U.S. patent application number 13/683432 was filed with the patent office on 2013-05-30 for light-emitting element, light-emitting device, electronic device, lighting device and organic compound.
This patent application is currently assigned to SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. The applicant listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Hiroshi Kadoma, Yasushi Kitano, Nobuharu Ohsawa, Satoshi Seo, Satoko Shitagaki.
Application Number | 20130134395 13/683432 |
Document ID | / |
Family ID | 48465988 |
Filed Date | 2013-05-30 |
United States Patent
Application |
20130134395 |
Kind Code |
A1 |
Kitano; Yasushi ; et
al. |
May 30, 2013 |
LIGHT-EMITTING ELEMENT, LIGHT-EMITTING DEVICE, ELECTRONIC DEVICE,
LIGHTING DEVICE AND ORGANIC COMPOUND
Abstract
A novel organic compound which can be used as a host material
for a phosphorescent compound is provided. A light-emitting element
containing the organic compound is provided. A light-emitting
device, an electronic device, and a lighting device each of which
includes the light-emitting element are provided. In the
light-emitting element including a light-emitting layer interposed
between a pair of electrodes, the light-emitting layer contains at
least an organic compound and a phosphorescent compound. In the
organic compound, a dibenzo[f,h]quinoxaline skeleton and an amino
group having two substituents are bonded to each other through an
arylene group. The substituents are separately an aryl group or a
heteroaryl group.
Inventors: |
Kitano; Yasushi; (Atsugi,
JP) ; Kadoma; Hiroshi; (Sagamihara, JP) ;
Shitagaki; Satoko; (Isehara, JP) ; Ohsawa;
Nobuharu; (Tochigi, JP) ; Seo; Satoshi;
(Sagamihara, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd.; |
Atsugi-shi |
|
JP |
|
|
Assignee: |
SEMICONDUCTOR ENERGY LABORATORY
CO., LTD.
Atsugi-shi
JP
|
Family ID: |
48465988 |
Appl. No.: |
13/683432 |
Filed: |
November 21, 2012 |
Current U.S.
Class: |
257/40 |
Current CPC
Class: |
H01L 51/0032 20130101;
H01L 51/0072 20130101; H01L 51/0074 20130101; H01L 51/5016
20130101; H01L 51/0059 20130101; H01L 51/0085 20130101 |
Class at
Publication: |
257/40 |
International
Class: |
H01L 51/00 20060101
H01L051/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 25, 2011 |
JP |
2011-258031 |
Claims
1. A light-emitting element comprising: a light-emitting layer
interposed between a pair of electrodes, wherein the light-emitting
layer contains a first organic compound and a phosphorescent
compound, wherein a dibenzo[f,h]quinoxaline skeleton and an amino
group having two substituents are bonded to each other through an
arylene group in the first organic compound, and wherein the
substituents are separately an aryl group or a heteroaryl
group.
2. The light-emitting element according to claim 1, wherein one of
the substituents includes a carbazole skeleton.
3. The light-emitting element according to claim 1, wherein a
2-position of the dibenzo[f,h]quinoxaline skeleton is bonded to the
amino group through an arylene group.
4. An organic compound represented by General Formula (G1):
##STR00070## wherein: each of R.sup.11 to R.sup.19 represents any
of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms; Ar.sup.1 represents a substituted or unsubstituted arylene
group having 6 to 13 carbon atoms; Ar.sup.2 represents a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms; .alpha. represents a substituted or unsubstituted phenylene
group or a substituted or unsubstituted biphenyldiyl group; n
represents 0 or 1; and A represents substituted or unsubstituted
9H-carbazol-9-yl group or a substituted or unsubstituted
9-aryl-9H-carbazol-3-yl group.
5. The organic compound according to claim 4, wherein: the organic
compound is represented by General Formula (G2-1), ##STR00071## and
each of R.sup.21 to R.sup.28 represents any of hydrogen, an alkyl
group having 1 to 4 carbon atoms, and a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms.
6. The organic compound according to claim 5, wherein .alpha. is a
substituted or unsubstituted para-phenylene group.
7. The organic compound according to claim 4, wherein: the organic
compound is represented by General Formula (G3-1), ##STR00072##
each of R.sup.31 to R.sup.37 represents any of hydrogen, an alkyl
group having 1 to 4 carbon atoms, and a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms; and Ar.sup.3
represents a substituted or unsubstituted aryl group having 6 to 13
carbon atoms.
8. The organic compound according to claim 7, wherein n is 1, and
wherein .alpha. is a substituted or unsubstituted para-phenylene
group.
9. The organic compound according to claim 4, wherein Ar.sup.1 is a
substituted or unsubstituted phenylene group or a substituted or
unsubstituted biphenyldiyl group.
10. The organic compound according to claim 4, wherein Ar.sup.1 is
a substituted or unsubstituted para-phenylene group.
11. The organic compound according to claim 5, wherein: the organic
compound is represented by General Formula (G2-2), ##STR00073## and
each of R.sup.41 to R.sup.44 and R.sup.51 to R.sup.54 represents
any of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms.
12. The organic compound according to claim 7, wherein: the organic
compound represented by General Formula (G3-2), ##STR00074## and
each of R.sup.41 to R.sup.44 and R.sup.51 to R.sup.54 represents
any of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms.
13. The organic compound according to claim 7, wherein: the organic
compound is represented by General Formula (G3-3), ##STR00075## and
each of R.sup.41 to R.sup.44 represents any of hydrogen, an alkyl
group having 1 to 4 carbon atoms, and a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms.
14. A light-emitting element comprising: a light-emitting layer
interposed between a pair of electrodes, wherein the light-emitting
layer contains the organic compound according to claim 4 and a
phosphorescent compound.
15. A light-emitting device comprising the light-emitting element
according to claim 14.
16. An electronic device comprising the light-emitting element
according to claim 14.
17. A lighting device comprising the light-emitting element
according to claim 14.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a light-emitting element, a
light-emitting device, an electronic device, a lighting device, and
an organic compound.
[0003] 2. Description of the Related Art
[0004] In recent years, research and development have been
extensively conducted on light-emitting elements using
electroluminescence (EL). In a basic structure of such a
light-emitting element, a layer containing a light-emitting
substance is interposed between a pair of electrodes. By applying
voltage to this element, light emission from the light-emitting
substance can be obtained.
[0005] Such a light-emitting element is self-luminous elements and
has advantages over liquid crystal displays, such as high
visibility of the pixels and no need of backlight; thus, such a
light-emitting element is thought to be suitable as a flat panel
display element. Besides, such a light-emitting element has
advantages in that it can be manufactured to be thin and
lightweight, and has very fast response speed.
[0006] Furthermore, since such a light-emitting element can be
formed in a film form, the light-emitting element makes it possible
to provide planar light emission; thus, a large-area element can be
easily formed. This feature is difficult to obtain with point light
sources typified by incandescent lamps and LEDs or linear light
sources typified by fluorescent lamps. Thus, the light-emitting
element also has great potential as a planar light source
applicable to a lighting device and the like.
[0007] Such light-emitting elements utilizing electroluminescence
can be broadly classified according to whether a light-emitting
substance is an organic compound or an inorganic compound. In the
case of an organic EL element in which a layer containing an
organic compound used as a light-emitting substance is provided
between a pair of electrodes, application of voltage to the
light-emitting element causes injection of electrons from a cathode
and holes from an anode into the layer containing the organic
compound having a light-emitting property and thus a current flows.
The injected electrons and holes then lead the organic compound to
its excited state, so that light emission is obtained from the
excited organic compound.
[0008] The excited state formed by an organic compound can be a
singlet excited state or a triplet excited state. Light emission
from the singlet excited state (S*) is called fluorescence, and
emission from the triplet excited state (T*) is called
phosphorescence. Further, the statistical generation ratio of S* to
T* in a light-emitting element is thought to be 1:3.
[0009] With a compound that can convert energy of a singlet excited
state into light emission (hereinafter, called a fluorescent
compound), only light emission from the singlet excited state
(fluorescence) is observed and that from the triplet excited state
(phosphorescence) is not observed, at room temperature.
Accordingly, the internal quantum efficiency (the ratio of the
number of generated photons to the number of injected carriers) of
a light-emitting element including the fluorescent compound is
assumed to have a theoretical limit of 25%, on the basis of
S*:T*=1:3.
[0010] In contrast, with a compound that can convert energy of a
triplet excited state into light emission (hereinafter, called a
phosphorescent compound), light emission from the triplet excited
state (phosphorescence) is observed. Further, since intersystem
crossing (i.e., transition from a singlet excited state to a
triplet excited state) easily occurs in a phosphorescent compound,
the internal quantum efficiency can be theoretically increased to
100%. In other words, higher emission efficiency can be obtained
than using a fluorescent compound. For this reason, light-emitting
elements using a phosphorescent compound have been under active
development recently so that high-efficiency light-emitting
elements can be achieved.
[0011] When a light-emitting layer of a light-emitting element is
formed using a phosphorescent compound described above, in order to
suppress concentration quenching or quenching due to
triplet-triplet annihilation in the phosphorescent compound, the
light-emitting layer is often formed such that the phosphorescent
compound is dispersed in a matrix of another compound. Here, the
compound as the matrix is called a host material, and the compound
dispersed in the matrix, such as a phosphorescent compound, is
called a guest material (dopant).
[0012] In the case where a phosphorescent compound is a guest
material, a host material needs to have higher triplet excited
energy (energy difference between a ground state and a triplet
excited state) than the phosphorescent compound. The host material
also needs to have properties of easily accepting and transporting
both holes and electrons (i.e., a bipolar property).
[0013] When a substance used as a host material has a bipolar
property, holes and electrons can be accepted efficiently; thus, a
light-emitting element in which such a host material is used in a
light-emitting layer can have lower driving voltage.
[0014] Furthermore, since singlet excitation energy (energy
difference between a ground state and a singlet excited state) is
higher than triplet excited energy, a substance that has high
triplet excited energy also has high singlet excitation energy.
Therefore the above substance that has high triplet excited energy
is also effective in a light-emitting element using a fluorescent
compound as a light-emitting substance.
[0015] As a host material which has a bipolar property and higher
triplet excited energy than a phosphorescent compound, a carbazole
derivative which has a heteroaromatic ring having, in the same
molecule, a carbazole skeleton with a hole-transport property and
an oxadiazole skeleton or a quinoxaline skeleton with an
electron-transport property which is the heteroaromatic ring is
disclosed (e.g., Patent Document 1).
REFERENCE
[0016] [Patent Document 1] Japanese Published Patent Application
No. 2010-241801
SUMMARY OF THE INVENTION
[0017] As reported in Patent Document 1, the development of a host
material for a phosphorescent compound has been actively conducted.
However, light-emitting elements still need to be improved in terms
of emission efficiency, reliability, emission characteristics,
synthesis efficiency, and cost, and better light-emitting elements
are expected to be developed.
[0018] In view of the above, an object of one embodiment of the
present invention is to provide a novel organic compound which can
be used as a host material for a phosphorescent compound. Another
object is to provide a light-emitting element containing the
organic compound.
[0019] Another object of one embodiment of the present invention is
to provide a light-emitting device, an electronic device, and a
lighting device each of which includes the light-emitting
element.
[0020] One embodiment of the present invention is a light-emitting
element which includes a light-emitting layer interposed between a
pair of electrodes. The light-emitting layer contains at least an
organic compound and a phosphorescent compound. In the organic
compound, a dibenzo[f,h]quinoxaline skeleton and an amino group
having two substituents are bonded to each other through an arylene
group. The substituents are separately an aryl group or a
heteroaryl group.
[0021] In the above structure, at least one of the substituents
preferably includes a carbazole skeleton. In addition, the
dibenzo[f,h]quinoxaline skeleton is a 2-position substituted
skeleton; the 2-position of the dibenzo[f,h]quinoxaline skeleton is
preferably bonded to the amino group through the arylene group.
[0022] The dibenzo[f,h]quinoxaline skeleton which has a high
electron-transport property and the amino group having two
substituents which has a high hole-transport property are bonded to
each other through the arylene group in the organic compound, so
that the organic compound can have a bipolar property. When the
organic compound has a bipolar property, holes and electrons can be
accepted efficiently. Further, the organic compound has a high
triplet excited energy level (T.sub.1 level). Thus, the
light-emitting element in which such an organic compound is used in
the light-emitting layer can have lower driving voltage.
[0023] Another embodiment of the present invention is a
light-emitting element which includes a light-emitting layer
interposed between a pair of electrodes. The light-emitting layer
contains at least an organic compound and a phosphorescent
compound. The organic compound can be represented by General
Formula (G1). Note that the organic compound represented by General
Formula (G1) is a useful, novel compound and is one embodiment of
the present invention.
##STR00001##
[0024] In General Formula (G1), R.sup.11 to R.sup.19 separately
represent hydrogen, an alkyl group having 1 to 4 carbon atoms, or a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms. Ar.sup.1 represents a substituted or unsubstituted arylene
group having 6 to 13 carbon atoms. Ar.sup.2 represents a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms. In addition, .alpha. represents a substituted or
unsubstituted phenylene group or a substituted or unsubstituted
biphenyldiyl group; n represents 0 or 1; A represents substituted
or unsubstituted 9H-carbazol-9-yl group or a substituted or
unsubstituted 9-aryl-9H-carbazol-3-yl group.
[0025] Another embodiment of the present invention is a
light-emitting element which includes a light-emitting layer
interposed between a pair of electrodes. The light-emitting layer
contains at least an organic compound and a phosphorescent
compound. The organic compound can be represented by General
Formula (G2-1). Note that the organic compound represented by
General Formula (G2-1) is a useful, novel compound and is one
embodiment of the present invention.
##STR00002##
[0026] In General Formula (G2-1), R.sup.11 to R.sup.19 and R.sup.21
to R.sup.28 separately represent hydrogen, an alkyl group having 1
to 4 carbon atoms, or a substituted or unsubstituted aryl group
having 6 to 13 carbon atoms. Ar.sup.1 represents a substituted or
unsubstituted arylene group having 6 to 13 carbon atoms. Ar.sup.2
represents a substituted or unsubstituted aryl group having 6 to 13
carbon atoms. In addition, .alpha. represents a substituted or
unsubstituted phenylene group or a substituted or unsubstituted
biphenyldiyl group.
[0027] Another embodiment of the present invention is a
light-emitting element which includes a light-emitting layer
interposed between a pair of electrodes. The light-emitting layer
contains at least an organic compound and a phosphorescent
compound. The organic compound can be represented by General
Formula (G3-1). Note that the organic compound represented by
General Formula (G3-1) is a useful, novel compound and is one
embodiment of the present invention.
##STR00003##
[0028] In General Formula (G3-1), R.sup.11 to R.sup.19 and R.sup.31
to R.sup.37 separately represent hydrogen, an alkyl group having 1
to 4 carbon atoms, or a substituted or unsubstituted aryl group
having 6 to 13 carbon atoms. Ar.sup.1 represents a substituted or
unsubstituted arylene group having 6 to 13 carbon atoms. Ar.sup.2
and Ar.sup.3 separately represent a substituted or unsubstituted
aryl group having 6 to 13 carbon atoms. In addition, a represents a
substituted or unsubstituted phenylene group or a substituted or
unsubstituted biphenyldiyl group; n represents 0 or 1.
[0029] In each of the organic compounds represented by General
Formulae (G2-1) and (G3-1), Ar.sup.1 is preferably a substituted or
unsubstituted phenylene group or a substituted or unsubstituted
biphenyldiyl group.
[0030] In the organic compound represented by General Formula
(G2-1), .alpha. is preferably a substituted or unsubstituted
para-phenylene group. In the organic compound represented by
General Formula (G3-1), n is preferably 1 and .alpha. is preferably
a substituted or unsubstituted para-phenylene group. In each of the
organic compounds represented by General Formulae (G2-1) and
(G3-1), Ar.sup.1 is preferably a substituted or unsubstituted
para-phenylene group. In other words, the organic compounds
represented by General Formulae (G2-1) and (G3-1) can be
represented by General Formulae (G2-2) and (G3-2),
respectively.
[0031] Thus, another embodiment of the present invention is a
light-emitting element which includes a light-emitting layer
interposed between a pair of electrodes. The light-emitting layer
contains at least an organic compound and a phosphorescent
compound. The organic compound can be represented by General
Formula (G2-2). Note that the organic compound represented by
General Formula (G2-2) is a useful, novel compound and is one
embodiment of the present invention.
##STR00004##
[0032] In General Formula (G2-2), R.sup.11 to R.sup.19, R.sup.21 to
R.sup.28, R.sup.41 to R.sup.44 and R.sup.51 to R.sup.54 separately
represent hydrogen, an alkyl group having 1 to 4 carbon atoms, or a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms. Ar.sup.2 represents a substituted or unsubstituted aryl
group having 6 to 13 carbon atoms.
[0033] Another embodiment of the present invention is a
light-emitting element which includes a light-emitting layer
interposed between a pair of electrodes. The light-emitting layer
contains at least an organic compound and a phosphorescent
compound. The organic compound can be represented by General
Formula (G3-2). Note that the organic compound represented by
General Formula (G3-2) is a useful, novel compound and is one
embodiment of the present invention.
##STR00005##
[0034] In General Formula (G3-2), R.sup.11 to R.sup.19, R.sup.31 to
R.sup.37, R.sup.41 to R.sup.44, and R.sup.51 to R.sup.54 separately
represent hydrogen, an alkyl group having 1 to 4 carbon atoms, or a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms. Ar.sup.2 and Ar.sup.3 separately represent a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms.
[0035] Another embodiment of the present invention is a
light-emitting element which includes a light-emitting layer
interposed between a pair of electrodes. The light-emitting layer
contains at least an organic compound and a phosphorescent
compound. The organic compound can be represented by General
Formula (G3-3). Note that the organic compound represented by
General Formula (G3-3) is a useful, novel compound and is one
embodiment of the present invention.
##STR00006##
[0036] In General Formula (G3-3), R.sup.11 to R.sup.19, R.sup.31 to
R.sup.37, and R.sup.41 to R.sup.44 separately represent hydrogen,
an alkyl group having 1 to 4 carbon atoms, or a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms. Ar.sup.2 and
Ar.sup.3 separately represent a substituted or unsubstituted aryl
group having 6 to 13 carbon atoms.
[0037] A light-emitting device, an electronic device, and a
lighting device each using the above light-emitting element are
also included in the scope of the present invention. Note that the
light-emitting device in this specification includes an image
display device and a light source. In addition, the light-emitting
device includes, in its category, all of a module in which a
connector such as a flexible printed circuit (FPC) or a tape
carrier package (TCP) is connected to a panel, a module in which a
printed wiring board is provided on the tip of a TCP, and a module
in which an integrated circuit (IC) is directly mounted on a
light-emitting element by a chip on glass (COG) method.
[0038] According to one embodiment of the present invention, a
novel organic compound which can be used as a host material for a
phosphorescent compound can be provided. A light-emitting element
containing the organic compound can also be provided. A
light-emitting element which has low driving voltage and high
current efficiency can also be provided. According to another
embodiment of the present invention, the use of the light-emitting
element makes it possible to provide a light-emitting device, an
electronic device, and a lighting device which have lower power
consumption.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 illustrates a light-emitting element of one
embodiment of the present invention.
[0040] FIG. 2 illustrates a light-emitting element of one
embodiment of the present invention.
[0041] FIGS. 3A and 3B each illustrate a light-emitting element of
one embodiment of the present invention.
[0042] FIG. 4 illustrates light-emitting elements of one embodiment
of the present invention.
[0043] FIGS. 5A and 5B illustrate a light-emitting device of one
embodiment of the present invention.
[0044] FIGS. 6A to 6D illustrate electronic devices of one
embodiment of the present invention.
[0045] FIGS. 7A-1 to 7B illustrate electronic devices of one
embodiment of the present invention.
[0046] FIGS. 8A to 8C illustrate lighting devices of one embodiment
of the present invention.
[0047] FIGS. 9A and 9B are .sup.1H NMR charts of
4-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylam-
ine (abbreviation: PCBAPDBq).
[0048] FIGS. 10A and 10B show an absorption spectrum and an
emission spectrum, respectively, of PCBAPDBq in a toluene solution
of PCBAPDBq.
[0049] FIGS. 11A and 11B show an absorption spectrum and an
emission spectrum, respectively, of a thin film of PCBAPDBq.
[0050] FIG. 12 is illustrates light-emitting elements of
Examples.
[0051] FIG. 13 shows luminance versus current density
characteristics of a light-emitting element 1.
[0052] FIG. 14 shows luminance versus voltage characteristics of
the light-emitting element 1.
[0053] FIG. 15 shows current efficiency versus luminance
characteristics of the light-emitting element 1.
[0054] FIG. 16 shows current versus voltage characteristics of the
light-emitting element 1.
[0055] FIG. 17 shows an emission spectrum of the light-emitting
element 1.
[0056] FIG. 18 shows normalized luminance versus driving time
characteristics of the light-emitting element 1.
[0057] FIG. 19 shows luminance versus current density
characteristics of a light-emitting element 2.
[0058] FIG. 20 shows luminance versus voltage characteristics of
the light-emitting element 2.
[0059] FIG. 21 shows current efficiency versus luminance
characteristics of the light-emitting element 2.
[0060] FIG. 22 shows current versus voltage characteristics of the
light-emitting element 2.
[0061] FIG. 23 shows an emission spectrum of the light-emitting
element 2.
[0062] FIG. 24 shows normalized luminance versus driving time
characteristics of the light-emitting element 2.
[0063] FIG. 25 shows the concept of an exciplex.
[0064] FIGS. 26A and 26B are .sup.1H NMR charts of
3-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9H-carbazol-9-yl)triphenylamine
(abbreviation: mYGAPDBq).
[0065] FIGS. 27A and 27B show an absorption spectrum and an
emission spectrum, respectively, of mYGAPDBq in a toluene solution
of mYGAPDBq.
[0066] FIGS. 28A and 28B show an absorption spectrum and an
emission spectrum, respectively, of a thin film of mYGAPDBq.
[0067] FIG. 29 shows results of LC/MS analysis of mYGAPDBq.
[0068] FIG. 30 shows luminance versus current density
characteristics of a light-emitting element 3 and a light-emitting
element 4.
[0069] FIG. 31 shows luminance versus voltage characteristics of
the light-emitting element 3 and the light-emitting element 4.
[0070] FIG. 32 shows current efficiency versus luminance
characteristics of the light-emitting element 3 and the
light-emitting element 4.
[0071] FIG. 33 shows current versus voltage characteristics of the
light-emitting element 3 and the light-emitting element 4.
[0072] FIG. 34 shows normalized luminance versus driving time
characteristics of the light-emitting element 3 and the
light-emitting element 4.
[0073] FIG. 35 shows luminance versus current density
characteristics of a light-emitting element 5 and a light-emitting
element 6.
[0074] FIG. 36 shows luminance versus voltage characteristics of
the light-emitting element 5 and the light-emitting element 6.
[0075] FIG. 37 shows current efficiency versus luminance
characteristics of the light-emitting element 5 and the
light-emitting element 6.
[0076] FIG. 38 shows current versus voltage characteristics of the
light-emitting element 5 and the light-emitting element 6.
[0077] FIG. 39 shows normalized luminance versus driving time
characteristics of the light-emitting element 5 and the
light-emitting element 6.
[0078] FIG. 40 shows luminance versus current density
characteristics of a light-emitting element 7.
[0079] FIG. 41 shows luminance versus voltage characteristics of
the light-emitting element 7.
[0080] FIG. 42 shows current efficiency versus luminance
characteristics of the light-emitting element 7.
[0081] FIG. 43 shows current versus voltage characteristics of the
light-emitting element 7.
DETAILED DESCRIPTION OF THE INVENTION
[0082] Hereinafter, embodiments of the present invention will be
described in detail with reference to the accompanying drawings.
Note that the present invention is not limited to the following
description, and it will be easily understood by those skilled in
the art that the mode and detail can be modified in various ways
without departing from the spirit and scope of the present
invention. Therefore, the present invention should not be construed
as being limited to the description in the following
embodiments.
Embodiment 1
[0083] In this embodiment, a structure of a light-emitting element,
in which a light-emitting layer is interposed between a pair of
electrodes and contains at least an organic compound and a
phosphorescent compound, will be described with reference to FIG.
1.
[0084] In a light-emitting element described in this embodiment, as
illustrated in FIG. 1, an EL layer 102 including a light-emitting
layer 113 is provided between a pair of electrodes (a first
electrode 101 and a second electrode 103), and the EL layer 102
includes a hole-injection layer 111, a hole-transport layer 112, an
electron-transport layer 114, an electron-injection layer 115, a
charge-generation layer 116, and the like in addition to the
light-emitting layer 113. Note that in this embodiment, the first
electrode 101 is used as an anode and the second electrode 103 is
used as a cathode. The first electrode 101 is formed over a
substrate 100. A glass substrate or the like can be used as the
substrate 100.
[0085] When voltage is applied to such a light-emitting element, a
hole injected from the first electrode 101 side and an electron
injected from the second electrode 103 side recombine in the
light-emitting layer 113, whereby a phosphorescent compound is
excited. Then, light is emitted when the phosphorescent compound in
the excited state returns to the ground state. Thus, in one
embodiment of the present invention, the phosphorescent compound
functions as a light-emitting substance in the light-emitting
element.
[0086] The light-emitting layer 113 contains at least an organic
compound and a phosphorescent compound. In the organic compound, a
dibenzo[f,h]quinoxaline skeleton and an amino group having two
substituents are bonded to each other through an arylene group. The
substituents are separately an aryl group or a heteroaryl
group.
[0087] The dibenzo[f,h]quinoxaline skeleton which has a high
electron-transport property and the amino group having two
substituents which has a high hole-transport property are bonded to
each other through the arylene group in the organic compound, so
that the organic compound can have a bipolar property. The organic
compound also has a high T.sub.1 level and thus can efficiently
accept holes and electrons. Thus, the light-emitting element
including the light-emitting layer 113 containing such an organic
compound can have lower driving voltage.
[0088] The hole-injection layer 111 included in the EL layer 102 is
a layer containing a substance having a high hole-transport
property and an acceptor substance. When electrons are extracted
from the substance having a high hole-transport property owing to
the acceptor substance, holes are generated. Thus, holes are
injected from the hole-injection layer 111 into the light-emitting
layer 113 through the hole-transport layer 112.
[0089] The charge-generation layer 116 is a layer containing a
substance having a high hole-transport property and an acceptor
substance. Electrons are extracted from the substance having a high
hole-transport property owing to the acceptor substance, and the
extracted electrons are injected from the electron-injection layer
115 having an electron-injection property into the light-emitting
layer 113 through the electron-transport layer 114.
[0090] A specific example in which the light-emitting element
described in this embodiment is manufactured is described
below.
[0091] For the first electrode 101 and the second electrode 103, a
metal, an alloy, an electrically conductive compound, a mixture
thereof, and the like can be used. Specifically, indium oxide-tin
oxide (ITO: indium tin oxide), indium oxide-tin oxide containing
silicon or silicon oxide, indium oxide-zinc oxide, indium oxide
containing tungsten oxide and zinc oxide, gold (Au), platinum (Pt),
nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron
(Fe), cobalt (Co), copper (Cu), palladium (Pd), and titanium (Ti)
can be used. In addition, an element belonging to Group 1 or Group
2 of the periodic table, for example, an alkali metal such as
lithium (Li) or cesium (Cs), an alkaline earth metal such as
magnesium (Mg), calcium (Ca), or strontium (Sr), an alloy
containing such an element (e.g., MgAg or AlLi), a rare earth metal
such as europium (Eu) or ytterbium (Yb), an alloy containing such
an element, graphene, and the like can be used. The first electrode
101 and the second electrode 103 can be formed by, for example, a
sputtering method, an evaporation method (including a vacuum
evaporation method), or the like.
[0092] Examples of the substance having a high hole-transport
property which is used for the hole-injection layer 111, the
hole-transport layer 112, and the charge-generation layer 116
include aromatic amine compounds such as
4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB
or .alpha.-NPD),
N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine
(abbreviation: TPD), 4,4',4''-tris(carbazol-9-yl)triphenylamine
(abbreviation: TCTA),
4,4',4''-tris(N,N-diphenylamino)triphenylamine (abbreviation:
TDATA),
4,4',4''-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine
(abbreviation: MTDATA), and
4,4'-bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl
(abbreviation: BSPB);
3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole
(abbreviation: PCzPCA1);
3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole
(abbreviation: PCzPCA2); and
3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole
(abbreviation: PCzPCN1). Other examples include carbazole compounds
such as 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP),
1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), and
9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation:
CzPA); dibenzothiophene compounds such as
4,4',4''-(1,3,5-benzenetriyl)tri(dibenzothiophene) (abbreviation:
DBT3P-II); dibenzofuran compounds such as
1,3,5-tri(dibenzofuran-4-yl)-benzene (abbreviation: DBF3P-II); and
condensed-ring compounds such as
9-[3,5-di(phenanthren-9-yl)-phenyl]-phenanthrene (abbreviation:
Pn3P). The substances given here are mainly ones having a hole
mobility of 10.sup.-6 cm.sup.2/Vs or higher. Note that any other
substance may be used as long as the substance has a hole-transport
property higher than an electron-transport property.
[0093] The organic compound of one embodiment of the present
invention has a hole-transport property and thus can also be used
for the hole-transport layer 112.
[0094] Further, a high molecular compound such as
poly(N-vinylcarbazole) (abbreviation: PVK),
poly(-vinyltriphenylamine) (abbreviation: PVTPA),
poly[N-(4-{N'-[4-(4-diphenylamino)phenyl]phenyl-N'-phenylamino}phenyl)met-
hacrylamide] (abbreviation: PTPDMA), or
poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine]
(abbreviation: Poly-TPD) can be used.
[0095] As each of the hole-injection layer 111 and the
charge-generation layer 116, a layer in which any of the substances
having a high hole-transport property given above and a substance
having an acceptor property are mixed is preferably used, in which
case a favorable carrier-injection property is obtained. As
examples of the acceptor substance to be used, a transition metal
oxide and an oxide of a metal belonging to any of Groups 4 to 8 of
the periodic table can be given. Specifically, molybdenum oxide is
particularly preferable.
[0096] The light-emitting layer 113 contains a phosphorescent
compound as a guest material which serves as a light-emitting
substance and is formed using a substance having higher triplet
excited energy than the phosphorescent compound as a host
material.
[0097] Here, the organic compound of one embodiment of the present
invention can be used as the host material. In the organic compound
of one embodiment of the present invention, a
dibenzo[f,h]quinoxaline skeleton and an amino group having two
substituents are bonded to each other through an arylene group. The
substituents are separately an aryl group or a heteroaryl group. At
least one of the substituents preferably includes a carbazole
skeleton. The dibenzo[f,h]quinoxaline skeleton is a 2-position
substituted skeleton; the 2-position of the dibenzo[f,h]quinoxaline
skeleton is preferably bonded to an amino group through an arylene
group.
[0098] In other words, the organic compound is an organic compound
represented by General Formula (G1). The organic compound
represented by General Formula (G1) is one embodiment of the
present invention.
##STR00007##
[0099] In General Formula (G1), R.sup.11 to R.sup.19 separately
represent hydrogen, an alkyl group having 1 to 4 carbon atoms, or a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms. Ar.sup.1 represents a substituted or unsubstituted arylene
group having 6 to 13 carbon atoms. Ar.sup.2 represents a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms. In addition, .alpha. represents a substituted or
unsubstituted phenylene group or a substituted or unsubstituted
biphenyldiyl group; n represents 0 or 1; A represents substituted
or unsubstituted 9H-carbazol-9-yl group or a substituted or
unsubstituted 9-aryl-9H-carbazol-3-yl group.
[0100] Specifically, an organic compound represented by General
Formula (G2-1) is preferable as the organic compound represented by
General Formula (G1). Further, the organic compound represented by
General Formula (G2-1) is one embodiment of the present
invention.
##STR00008##
[0101] In General Formula (G2-1), R.sup.11 to R.sup.19 and R.sup.21
to R.sup.28 separately represent hydrogen, an alkyl group having 1
to 4 carbon atoms, or a substituted or unsubstituted aryl group
having 6 to 13 carbon atoms. Ar.sup.1 represents a substituted or
unsubstituted arylene group having 6 to 13 carbon atoms. Ar.sup.2
represents a substituted or unsubstituted aryl group having 6 to 13
carbon atoms. In addition, .alpha. represents a substituted or
unsubstituted phenylene group or a substituted or unsubstituted
biphenyldiyl group.
[0102] Specifically, an organic compound represented by General
Formula (G3-1) is preferable as the organic compound represented by
General Formula (G1). Further, the organic compound represented by
General Formula (G3-1) is one embodiment of the present
invention.
##STR00009##
[0103] In General Formula (G3-1), R.sup.11 to R.sup.19 and R.sup.31
to R.sup.37 separately represent hydrogen, an alkyl group having 1
to 4 carbon atoms, or a substituted or unsubstituted aryl group
having 6 to 13 carbon atoms. Ar.sup.1 represents a substituted or
unsubstituted arylene group having 6 to 13 carbon atoms. Ar.sup.2
and Ar.sup.3 represent a substituted or unsubstituted aryl group
having 6 to 13 carbon atoms. In addition, .alpha. represents a
substituted or unsubstituted phenylene group or a substituted or
unsubstituted biphenyldiyl group; n represents 0 or 1.
[0104] In each of the organic compounds represented by General
Formulae (G2-1) and (G3-1), Ar.sup.1 is preferably a substituted or
unsubstituted phenylene group or a substituted or unsubstituted
biphenyldiyl group.
[0105] In the organic compound represented by General Formula
(G2-1), .alpha. is preferably a substituted or unsubstituted
para-phenylene group. In the organic compound represented by
General Formula (G3-1), n is preferably 1 and .alpha. is preferably
a substituted or unsubstituted para-phenylene group. In each of the
organic compounds represented by General Formulae (G2-1) and
(G3-1), Ar.sup.1 is preferably a substituted or unsubstituted
para-phenylene group. In other words, the organic compounds
represented by General Formulae (G2-1) and (G3-1) can be
represented by General Formulae (G2-2) and (G3-2), respectively.
Further, the organic compounds represented by General Formulae
(G2-2) and (G3-2) are each one embodiment of the present
invention.
##STR00010##
[0106] In General Formula (G2-2), R.sup.11 to R.sup.19, R.sup.21 to
R.sup.28, R.sup.41 to R.sup.44 and R.sup.51 to R.sup.54 separately
represent hydrogen, an alkyl group having 1 to 4 carbon atoms, or a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms. Ar.sup.2 represents a substituted or unsubstituted aryl
group having 6 to 13 carbon atoms.
##STR00011##
[0107] In General Formula (G3-2), R.sup.11 to R.sup.19, R.sup.31 to
R.sup.37, R.sup.41 to R.sup.44, and R.sup.51 to R.sup.54 separately
represent hydrogen, an alkyl group having 1 to 4 carbon atoms, or a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms. Ar.sup.2 and Ar.sup.3 separately represent a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms.
[0108] Specifically, organic compounds represented by General
Formulae (G1) and (G3-1) are preferable as the organic compound
represented by General Formula (G3-3). Further, the organic
compound represented by General Formula (G3-3) is one embodiment of
the present invention.
##STR00012##
[0109] In General Formula (G3-3), R.sup.11 to R.sup.19, R.sup.31 to
R.sup.37, and R.sup.41 to R.sup.44 separately represent hydrogen,
an alkyl group having 1 to 4 carbon atoms, or a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms. Ar.sup.2 and
Ar.sup.3 separately represent a substituted or unsubstituted aryl
group having 6 to 13 carbon atoms.
[0110] Specific examples of Ar.sup.1 in General Formulae (G1),
(G2-1), and (G3-1) are substituents represented by Structural
Formulae (Ar-1) to (Ar-15).
##STR00013## ##STR00014## ##STR00015##
[0111] Specific examples of Ar.sup.2 in General Formulae (G1),
(G2-1), (G3-1), (G2-2), (G3-2), and (G3-3) and Ar.sup.a in General
Formulae (G3-1), (G3-2), and (G3-3) are substituents represented by
Structural Formulae (Ar-16) to (Ar-29).
##STR00016## ##STR00017## ##STR00018##
[0112] Specific examples of R.sup.11 to R.sup.19 in General
Formulae (G1), (G2-1), (G3-1), (G2-2), (G3-2), and (G3-3), R.sup.21
to R.sup.28 in General Formulae (G2-1) and (G2-2), R.sup.31 to
R.sup.37 in General Formulae (G3-1), (G3-2), and (G3-3), R.sup.41
to R.sup.44 in General Formulae (G2-2), (G3-2), and (G3-3), and
R.sup.51 to R.sup.54 in General Formulae (G2-2) and (G3-2) are
substituents represented by Structural Formulae (R-1) to
(R-23).
##STR00019## ##STR00020## ##STR00021##
[0113] Specific examples of the organic compounds represented by
General Formulae (G1), (G2-1), (G3-1), (G2-2), (G3-2), and (G3-3)
are organic compounds represented by Structural Formulae (100) to
(157) and Structural Formulae (200) to (235).
##STR00022## ##STR00023## ##STR00024## ##STR00025## ##STR00026##
##STR00027## ##STR00028## ##STR00029## ##STR00030## ##STR00031##
##STR00032## ##STR00033## ##STR00034## ##STR00035## ##STR00036##
##STR00037## ##STR00038## ##STR00039## ##STR00040## ##STR00041##
##STR00042## ##STR00043## ##STR00044## ##STR00045## ##STR00046##
##STR00047## ##STR00048## ##STR00049## ##STR00050## ##STR00051##
##STR00052## ##STR00053##
[0114] A variety of reactions can be applied to a synthesis method
of the organic compound of one embodiment of the present invention.
For example, synthesis reactions described below enable the
synthesis of the organic compound represented by General Formula
(G1). Note that the synthesis method of the organic compound is not
limited to the synthesis method below.
<Synthesis Method of Organic Compound Represented by General
Formula (G1)>
[0115] First, Synthesis Scheme (A-1) is described below. As shown
in Synthesis Scheme (A-1), a halide of a dibenzo[f,h]quinoxaline
derivative (Compound 1) and an organoboron compound of an amine
derivative or boronic acid of an amine derivative (Compound 2) are
coupled by Suzuki-Miyaura Coupling, whereby the organic compound
represented by General Formula (G1), which is a target compound,
can be synthesized.
##STR00054##
[0116] In Synthesis Scheme (A-1), R.sup.11 to R.sup.19 separately
represent hydrogen, an alkyl group having 1 to 4 carbon atoms, or a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms, and R.sup.60 and R.sup.61 separately represent any of
hydrogen or an alkyl group having 1 to 6 carbon atoms. In the
synthesis scheme (A-1), R.sup.60 and R.sup.61 may be bonded to each
other to form a ring. X.sup.1 represents a halogen or a triflate
group. Ar.sup.1 represents a substituted or unsubstituted arylene
group having 6 to 13 carbon atoms. Ar.sup.2 represents a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms. In addition, .alpha. represents a substituted or
unsubstituted phenylene group or a substituted or unsubstituted
biphenyldiyl group; n represents 0 or 1; A represents substituted
or unsubstituted 9H-carbazol-9-yl group or a substituted or
unsubstituted 9-aryl-9H-carbazol-3-yl group.
[0117] Examples of a palladium catalyst which can be used in
Synthesis Scheme (A-1) include, but not limited to, palladium(II)
acetate, tetrakis(triphenylphosphine)palladium(0),
bis(triphenylphosphine)palladium(II) dichloride, and the like.
[0118] Examples of a ligand of the palladium catalyst which can be
used in Synthesis Scheme (A-1) include, but not limited to,
tri(ortho-tolyl)phosphine, triphenylphosphine,
tricyclohexylphosphine, and the like.
[0119] Examples of a base which can be used in Synthesis Scheme
(A-1) include, but not limited to, an organic base such as sodium
tert-butoxide and an inorganic base such as potassium carbonate and
sodium carbonate.
[0120] Examples of a solvent which can be used in Synthesis Scheme
(A-1) include, but not limited to, a mixed solvent of toluene and
water; a mixed solvent of toluene, alcohol such as ethanol, and
water; a mixed solvent of xylene and water; a mixed solvent of
xylene, alcohol such as ethanol, and water; a mixed solvent of
benzene and water; a mixed solvent of benzene, alcohol such as
ethanol, and water; and a mixed solvent of an ether such as
ethylene glycol dimethyl ether and water. Note that a mixed solvent
of toluene and water; a mixed solvent of toluene, ethanol, and
water; or a mixed solvent of water and ether such as ethylene
glycol dimethyl ether is more preferable.
[0121] As the coupling reaction shown in Synthesis Scheme (A-1),
the Suzuki-Miyaura reaction using the organoboron compound or the
boronic acid represented by Compound 2 may be replaced with a cross
coupling reaction using an organoaluminum compound, an
organozirconium compound, an organozinc compound, an organotin
compound, or the like. However, the present invention is not
limited thereto.
[0122] In the coupling reaction shown in Synthesis Scheme (A-1), an
organoboron compound of a dibenzo[f,h]quinoxaline derivative or
boronic acid of a dibenzo[f,h]quinoxaline derivative may be coupled
with a halide of an amine derivative or an amine derivative which
has a triflate group as a substituent by the Suzuki-Miyaura
reaction.
[0123] Further, as shown in Synthesis Scheme (A-2), a halide of a
dibenzo[f,h]quinoxaline derivative (Compound 3) and an amine
derivative (Compound 4) are coupled using a metal catalyst, a
metal, or a metal compound in the presence of a base, whereby the
organic compound represented by General Formula (G1), which is a
target compound, can be synthesized.
##STR00055##
[0124] In Synthesis Scheme (A-2), R.sup.11 to R.sup.19 separately
represent hydrogen, an alkyl group having 1 to 4 carbon atoms, or a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms. X.sup.2 represents a halogen or a triflate group, and the
halogen is preferably iodine or bromine. Ar.sup.1 represents a
substituted or unsubstituted arylene group having 6 to 13 carbon
atoms. Ar.sup.2 represents a substituted or unsubstituted aryl
group having 6 to 13 carbon atoms. In addition, .alpha. represents
a substituted or unsubstituted phenylene group or a substituted or
unsubstituted biphenyldiyl group; n represents 0 or 1; A represents
substituted or unsubstituted 9H-carbazol-9-yl group or a
substituted or unsubstituted 9-aryl-9H-carbazol-3-yl group.
[0125] In the case where the Buchwald-Hartwig reaction is performed
in Synthesis Scheme (A-2), examples of a palladium catalyst which
can be used include, but not limited to,
bis(dibenzylideneacetone)palladium(0) and palladium(II)
acetate.
[0126] Examples of a ligand in the palladium catalyst which can be
used in Synthesis Scheme (A-2) include, but not limited to,
tri(tert-butyl)phosphine, tri(n-hexyl)phosphine, and
tricyclohexylphosphine.
[0127] Examples of a base which can be used in Synthesis Scheme
(A-2) include, but not limited to, an organic base such as sodium
tert-butoxide and an inorganic base such as potassium
carbonate.
[0128] Examples of a solvent which can be used in Synthesis Scheme
(A-2) include, but not limited to, toluene, xylene, benzene, and
tetrahydrofuran.
[0129] Other than the Hartwig-Buchwald reaction, the Ullmann
reaction or the like may be used, and the reaction is not limited
to these.
[0130] Thus, the organic compound of one embodiment of the present
invention can be synthesized.
[0131] Note that in the case where the light-emitting layer 113
contains the above-described organic compound (host material) and a
phosphorescent compound (guest material), the light-emitting layer
113 can emit phosphorescence with high emission efficiency.
[0132] Although the light-emitting element containing a
phosphorescent compound is described in this embodiment, the
present invention is not limited thereto. In the organic compound
of one embodiment of the present invention, a
dibenzo[f,h]quinoxaline skeleton and an amino group having two
substituents are bonded to each other through an arylene group. The
substituents are separately an aryl group or a heteroaryl group.
Further, the organic compound of one embodiment of the present
invention has a high T.sub.1 level and thus also has a high singlet
excited energy level (S.sub.1 level). Thus, the organic compound of
one embodiment of the present invention can also be used as a host
material for a material emitting fluorescence in the visible light
region.
[0133] Plural kinds of substances can be used as the substances
(host materials) in which the light-emitting substance (guest
material) is dispersed. Thus, the light-emitting layer 113 may
contain a second host material (also referred to as assist
material) in addition to the organic compound of one embodiment of
the present invention. Note that the organic compound of one
embodiment of the present invention may be used as the second host
material (assist material).
[0134] Examples of the second host material include the materials
used for the hole-transport layer 112.
[0135] The electron-transport layer 114 is a layer containing a
substance having a high electron-transport property. For the
electron-transport layer 114, a metal complex such as Alq.sub.3,
tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq.sub.3),
bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation:
BeBq.sub.2), BAlq, Zn(BOX).sub.2, or
bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation:
Zn(BTZ).sub.2) can be used. Further, a heteroaromatic compound such
as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(abbreviation: PBD),
1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene
(abbreviation: OXD-7),
3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole
(abbreviation: TAZ),
3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole
(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),
bathocuproine (abbreviation: BCP), or
4,4'-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs) can
be used. Further, a high molecular compound such as
poly(2,5-pyridinediyl) (abbreviation: PPy),
poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)]
(abbreviation: PF-Py) or
poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2'-bipyridine-6,6'-diyl)]
(abbreviation: PF-BPy) can be used. The substances given here are
mainly ones having an electron mobility of 10.sup.-6 cm.sup.2/Vs or
higher. Note that any other substance may be used for the
electron-transport layer as long as the substance has an
electron-transport property higher than a hole-transport
property.
[0136] The electron-transport layer 114 is not limited to a single
layer, but may be a stack of two or more layers containing any of
the above substances. Further, the organic compound of one
embodiment of the present invention has an electron-transport
property and thus can also be used for the electron-transport layer
114.
[0137] The electron-injection layer 115 is a layer containing a
substance having a high electron-injection property. For the
electron-injection layer 115, an alkali metal, an alkaline earth
metal, or a compound thereof, such as lithium fluoride (LiF),
cesium fluoride (CsF), calcium fluoride (CaF.sub.2), or lithium
oxide (LiO.sub.x) can be used. A rare earth metal compound like
erbium fluoride (ErF.sub.3) can also be used. Any of the substances
for forming the electron-transport layer 114, which are given
above, can also be used.
[0138] Alternatively, a composite material in which an organic
compound and an electron donor (donor) are mixed may be used for
the electron-injection layer 115. Such a composite material is
excellent in an electron-injection property and an
electron-transport property because electrons are generation in the
organic compound by the electron donor. In this case, the organic
compound is preferably a material excellent in transporting the
generated electrons. Specifically, for example, the substances for
forming the electron-transport layer 114 (e.g., a metal complex and
a heteroaromatic compound), which are given above, can be used. As
the electron donor, a substance exhibiting an electron-donating
property to the organic compound may be used; specific examples
thereof include an alkali metal, an alkaline-earth metal, and a
rare earth metal, such as lithium, cesium, magnesium, calcium,
erbium, and ytterbium. Further, an alkali metal oxide or an
alkaline-earth metal oxide is preferable; examples thereof include
lithium oxide, calcium oxide, and barium oxide. A Lewis base such
as magnesium oxide can also be used. An organic compound such as
tetrathiafulvalene (abbreviation: TTF) can also be used.
[0139] Note that each of the above-described hole-injection layer
111, hole-transport layer 112, light-emitting layer 113,
electron-transport layer 114, electron-injection layer 115, and
charge-generation layer 116 can be formed by a method such as an
evaporation method (e.g., a vacuum evaporation method), an ink-jet
method, or a coating method.
[0140] In the above-described light-emitting element, current flows
due to a potential difference generated between the first electrode
101 and the second electrode 103 and holes and electrons recombine
in the EL layer 102, whereby light is emitted. Then, the emitted
light is extracted outside through one or both of the first
electrode 101 and the second electrode 103. Thus, one or both of
the first electrode 101 and the second electrode 103 are electrodes
having a light-transmitting property.
[0141] The above-described light-emitting element can emit
phosphorescence originating from the phosphorescent compound and
thus can have higher efficiency than a light-emitting element using
a fluorescent compound.
[0142] Note that although the light-emitting element described in
this embodiment is one structural example of a light-emitting
element, a light-emitting element having another structure which is
described in another embodiment can also be applied to a
light-emitting device of one embodiment of the present invention.
Further, as a light-emitting device including the above
light-emitting element, a passive matrix type light-emitting device
and an active matrix type light-emitting device can be
manufactured. It is also possible to manufacture a light-emitting
device with a microcavity structure including a light-emitting
element which is a different light-emitting element from the above
light-emitting elements as described in another embodiment. Each of
the above light-emitting devices is included in the present
invention.
[0143] Note that there is no particular limitation on the structure
of the TFT in the case of manufacturing the active matrix
light-emitting device. For example, a staggered TFT or an inverted
staggered TFT can be used as appropriate. Further, a driver circuit
formed over a TFT substrate may be formed using both of an n-type
TFT and a p-type TFT or only either an n-type TFT or a p-type TFT.
Furthermore, there is no particular limitation on the crystallinity
of a semiconductor film used for the TFT. For example, an amorphous
semiconductor film, a crystalline semiconductor film, an oxide
semiconductor film, or the like can be used.
[0144] Note that the structure described in this embodiment can be
combined as appropriate with any of the structures described in the
other embodiments.
Embodiment 2
[0145] In this embodiment, a light-emitting element which includes
a light-emitting layer containing a phosphorescent compound, an
organic compound of one embodiment of the present invention, and
two other organic compounds will be described with reference to
FIG. 2 and FIG. 25.
[0146] A light-emitting element described in this embodiment
includes an EL layer 203 between a pair of electrodes (a first
electrode 201 and a second electrode 202) as illustrated in FIG. 2.
Note that the EL layer 203 includes at least a light-emitting layer
204 and may include a hole-injection layer, a hole-transport layer,
an electron-transport layer, an electron-injection layer, a
charge-generation layer, and the like. Note that the substances
given in Embodiment 1 can be used for the hole-injection layer, the
hole-transport layer, the electron-transport layer, the
electron-injection layer, and the charge-generation layer. Note
that the first electrode 201 is used as an anode and the second
electrode 202 is used as a cathode in this embodiment.
[0147] The light-emitting layer 204 described in this embodiment
contains a phosphorescent compound 205, a first organic compound
206, and a second organic compound 207. Note that the
phosphorescent compound 205 is a guest material in the
light-emitting layer 204. Moreover, at least one of the first
organic compound 206 and the second organic compound 207 contains
the organic compound of one embodiment of the present invention,
and the one with a higher content than the other in the
light-emitting layer 204 is a host material in the light-emitting
layer 204.
[0148] When the light-emitting layer 204 has the structure in which
the guest material is dispersed in the host material, the
crystallization of the light-emitting layer can be suppressed.
Further, it is possible to suppress concentration quenching due to
high concentration of the guest material; thus, the light-emitting
element can have higher emission efficiency.
[0149] Note that it is preferable that a triplet excited energy
level (T.sub.1 level) of each of the first organic compound 206 and
the second organic compound 207 be higher than that of the
phosphorescent compound 205. This is because, when the T.sub.1
level of the first organic compound 206 (or the second organic
compound 207) is lower than that of the phosphorescent compound
205, the triplet excited energy of the phosphorescent compound 205,
which is to contribute to light emission, is quenched by the first
organic compound 206 (or the second organic compound 207) and
accordingly the emission efficiency decreases.
[0150] Here, for improvement in efficiency of energy transfer from
a host material to a guest material, Forster mechanism
(dipole-dipole interaction) and Dexter mechanism (electron exchange
interaction), which are known as mechanisms of energy transfer
between molecules, are considered. According to the mechanisms, it
is preferable that an emission spectrum of a host material
(fluorescence spectrum in energy transfer from a singlet excited
state, phosphorescence spectrum in energy transfer from a triplet
excited state) largely overlap with an absorption spectrum of a
guest material (specifically, spectrum in an absorption band on the
longest wavelength (lowest energy) side). However, in general, it
is difficult to obtain an overlap between a fluorescence spectrum
of a host material and an absorption spectrum in an absorption band
on the longest wavelength (lowest energy) side of a guest material.
The reason for this is as follows: if the fluorescence spectrum of
the host material overlaps with the absorption spectrum in the
absorption band on the longest wavelength (lowest energy) side of
the guest material, because a phosphorescence spectrum of the host
material is located on a longer wavelength (lower energy) side than
the fluorescence spectrum, the T.sub.1 level of the host material
becomes lower than the T.sub.1 level of the phosphorescent compound
and the above-described problem of quenching occurs; yet, when the
host material is designed in such a manner that the T.sub.1 level
of the host material is higher than the T.sub.1 level of the
phosphorescent compound to avoid the problem of quenching, the
fluorescence spectrum of the host material is shifted to the
shorter wavelength (higher energy) side, and thus the fluorescence
spectrum does not have any overlap with the absorption spectrum in
the absorption band on the longest wavelength (lowest energy) side
of the guest material. For this reason, in general, it is difficult
to obtain an overlap between a fluorescence spectrum of a host
material and an absorption spectrum in an absorption band on the
longest wavelength (lowest energy) side of a guest material so as
to maximize energy transfer from a singlet excited state of a host
material.
[0151] Thus, in this embodiment, a combination of the first organic
compound 206 and the second organic compound 207 preferably forms
an excited complex (also referred to as exciplex). Thus, in the
light-emitting layer 204, a fluorescence spectrum of the first
organic compound 206 and that of the second organic compound 207
are observed as an emission spectrum of the exciplex which is
located on a longer wavelength side. Moreover, when the first
organic compound 206 and the second organic compound 207 are
selected in such a manner that the emission spectrum of the
exciplex largely overlaps with the absorption spectrum of the guest
material (phosphorescent compound 205), energy transfer from a
singlet excited state can be maximized (see FIG. 25). Note that
also in the case of a triplet excited state, energy transfer from
the exciplex, not the host material, is assumed to occur. FIG. 25
shows a fluorescent spectrum 251 of the first organic compound 206
(or the second organic compound 207), a phosphorescent spectrum 252
of the first organic compound 206 (or the second organic compound
207), an absorption spectrum 253 of the phosphorescent compound
205, an emission spectrum 254 of the exciplex, and an absorption
band 255 positioned on the longest wavelength side.
[0152] As the phosphorescent compound 205, for example, a
phosphorescent iridium metal complex or the like can be used.
Examples of a material for blue light emission include [0153]
bis[2-(4',6'-difluorophenyl)pyridinato-N,C.sup.2']iridium(III)tetrakis(1--
pyrazolyl)borate (abbreviation: FIr6),
bis[2-(4',6'-difluorophenyl)pyridinato-N,C.sup.2']iridium(III)picolinate
(abbreviation: Flrpic), [0154]
bis{2-[3',5'-bis(trifluoromethyl)phenyl]pyridinato-N,C.sup.2'}iridium(III-
)picolinate (abbreviation: Ir(CF.sub.3ppy).sub.2(pic)), and [0155]
bis[2-(4',6'-difluorophenyl)pyridinato-N,C.sup.2']iridium(III)acetylaceto-
nate (abbreviation: FIr(acac)). Examples of a material for green
light emission include
tris(2-phenylpyridinato-N,C.sup.2')iridium(III) (abbreviation:
Ir(ppy).sub.3),
bis(2-phenylpyridinato-N,C.sup.2')iridium(III)acetylacetonate
(abbreviation: Ir(ppy).sub.2(acac)),
bis(1,2-diphenyl-1H-benzimidazolato)iridium(III)acetylacetonate
(abbreviation: Ir(pbi).sub.2(acac)),
bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation:
Ir(bzq).sub.2(acac)), and tris(benzo[h]quinolinato)iridium(III)
(abbreviation: Ir(bzq).sub.3). Examples of a material for yellow
light emission include [0156]
bis(2,4-diphenyl-1,3-oxazolato-N,C.sup.2')iridium(III)acetylacetonate
(abbreviation: Ir(dpo).sub.2(acac)),
bis[2-(4'-perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate
(abbreviation: Ir(p-PF-ph).sub.2(acac)), [0157]
bis(2-phenylbenzothiazolato-N,C.sup.2')iridium(III)acetylacetonate
(abbreviation: Ir(bt).sub.2(acac)), [0158]
(acetylacetonato)bis[2,3-bis(4-fluorophenyl)-5-methylpyrazinato]iridium(I-
II) (abbreviation: Ir(Fdppr-Me).sub.2(acac)), and [0159]
(acetylacetonato)bis{2-(4-methoxyphenyl)-3,5-dimethylpyrazinato}iridium(I-
II) (abbreviation: Ir(dmmoppr).sub.2(acac)). Examples of a material
for orange light emission include
tris(2-phenylquinolinato-N,C.sup.2')iridium(III) (abbreviation:
Ir(pq).sub.3), [0160]
bis(2-phenylquinolinato-N,C.sup.2')iridium(III)acetylacetonate
(abbreviation: Ir(pq).sub.2(acac)), [0161]
(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)
(abbreviation: Ir(mppr-Me).sub.2(acac)), and [0162]
(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)
(abbreviation: Ir(mppr-iPr).sub.2(acac)). Examples of a material
for red light emission include organometallic complexes such as
[0163]
bis[2-(2'-benzo[4,5-.alpha.]thienyl)pyridinato-N,C.sup.3']iridium(III)ace-
tylacetonate (abbreviation: Ir(btp).sub.2(acac)), [0164]
bis(1-phenylisoquinolinato-N,C.sup.2')iridium(III)acetylacetonate
(abbreviation: Ir(piq).sub.2(acac)),
(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)
(abbreviation: Ir(Fdpq).sub.2(acac)), [0165]
(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)
(abbreviation: Ir(tppr).sub.2(acac)),
(dipivaloylmethanato)bis(2,3,5-triphenylpyrazinato)iridium(III)
(abbreviation: Ir(tppr).sub.2(dpm)), and [0166]
2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinplatinum(II)
(abbreviation: PtOEP). Further, rare-earth metal complexes, such as
tris(acetylacetonato) (monophenanthroline)terbium(III)
(abbreviation: Tb(acac).sub.3(Phen)),
tris(1,3-diphenyl-1,3-propanedionato)
(monophenanthroline)europium(III) (abbreviation:
Eu(DBM).sub.3(Phen)), and
tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europ-
ium(III) (abbreviation: Eu(TTA).sub.3(Phen)), exhibit light
emission from rare-earth metal ions (electron transition between
different multiplicities), and thus can be used as phosphorescent
compounds. For the first organic compound 206 and the second
organic compound 207, a combination of a compound which easily
accepts electrons (a compound having an electron-trapping property)
and a compound which easily accepts holes (a compound having a
hole-trapping property) is preferably employed. Note that the
organic compound of one embodiment of the present invention can be
used as a compound which easily accepts holes.
[0167] Examples of a compound which easily accepts electrons
include [0168]
2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2mDBTPDBq-II),
2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2CzPDBq-III), [0169]
7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 7mDBTPDBq-II), and
6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 6mDBTPDBq-II).
[0170] The above-described combination of the first organic
compound 206 and the second organic compound 207 is an example of
the combination which enables an exciplex to be formed. The
combination is determined so that the emission spectrum of the
exciplex overlaps with the absorption spectrum of the
phosphorescent compound 205 and that the peak of the emission
spectrum of the exciplex has a longer wavelength than the peak of
the absorption spectrum of the phosphorescent compound 205.
[0171] Note that in the case where a compound which easily accepts
electrons and a compound which easily accepts holes are used for
the first organic compound 206 and the second organic compound 207,
carrier balance can be controlled by the mixture ratio of the
compounds. Specifically, the ratio of the first organic compound to
the second organic compound is preferably 1:9 to 9:1.
[0172] In the light-emitting element described in this embodiment,
energy transfer efficiency can be improved owing to energy transfer
utilizing an overlap between an emission spectrum of an exciplex
and an absorption spectrum of a phosphorescent compound;
accordingly, it is possible to achieve high external quantum
efficiency of a light-emitting element.
[0173] Note that in another structure of the present invention, the
light-emitting layer 204 can be formed using a host molecule having
a hole-trapping property and a host molecule having an
electron-trapping property as the two kinds of organic compounds
other than the phosphorescent compound 205 (guest material) so that
a phenomenon (guest coupled with complementary hosts: GCCH) occurs
in which holes and electrons are introduced to guest molecules
existing in the two kinds of host molecules and the guest molecules
are brought into an excited state.
[0174] At this time, the host molecule having a hole-trapping
property and the host molecule having an electron-trapping property
can be respectively selected from the above-described compounds
which easily accept holes and the above-described compounds which
easily accept electrons.
[0175] Note that although the light-emitting element described in
this embodiment is one structural example of a light-emitting
element, a light-emitting element having another structure which is
described in another embodiment can also be applied to a
light-emitting device that is one embodiment of the present
invention. Further, as a light-emitting device including the above
light-emitting element, a passive matrix type light-emitting device
and an active matrix type light-emitting device can be
manufactured. It is also possible to manufacture a light-emitting
device with a microcavity structure including a light-emitting
element which is a different light-emitting element from the above
light-emitting elements as described in another embodiment. Each of
the above light-emitting devices is included in the present
invention.
[0176] Note that there is no particular limitation on the structure
of the TFT in the case of manufacturing the active matrix
light-emitting device. For example, a staggered TFT or an inverted
staggered TFT can be used as appropriate. Further, a driver circuit
formed over a TFT substrate may be formed using both of an n-type
TFT and a p-type TFT or only either an n-type TFT or a p-type TFT.
Furthermore, there is no particular limitation on the crystallinity
of a semiconductor film used for the TFT. For example, an amorphous
semiconductor film, a crystalline semiconductor film, an oxide
semiconductor film, or the like can be used.
[0177] Note that the structure described in this embodiment can be
combined as appropriate with any of the structures described in the
other embodiments.
Embodiment 3
[0178] In this embodiment, as one embodiment of the present
invention, a light-emitting element (hereinafter referred to as
tandem light-emitting element) in which a charge generation layer
is provided between a plurality of EL layers will be described.
[0179] A light-emitting element described in this embodiment is a
tandem light-emitting element including a plurality of EL layers (a
first EL layer 302(1) and a second EL layer 302(2)) between a pair
of electrodes (a first electrode 301 and a second electrode 304),
as illustrated in FIG. 3A.
[0180] In this embodiment, the first electrode 301 functions as an
anode, and the second electrode 304 functions as a cathode. Note
that the first electrode 301 and the second electrode 304 can have
structures similar to those described in Embodiment 1. In addition,
although the plurality of EL layers (the first EL layer 302(1) and
the second EL layer 302(2)) may have structures similar to those
described in Embodiment 1 or 2, any of the EL layers may have a
structure similar to that described in Embodiment 1 or 2. In other
words, the structures of the first EL layer 302(1) and the second
EL layer 302(2) may be the same or different from each other and
can be similar to those described in Embodiment 1 or 2.
[0181] Further, a charge-generation layer 305 is provided between
the plurality of EL layers (the first EL layer 302(1) and the
second EL layer 302(2)). The charge generation layer 305 has a
function of injecting electrons into one of the EL layers and
injecting holes into the other of the EL layers when a voltage is
applied to the first electrode 301 and the second electrode 304. In
this embodiment, when voltage is applied such that the potential of
the first electrode 301 is higher than that of the second electrode
304, the charge-generation layer 305 injects electrons into the
first EL layer 302(1) and injects holes into the second EL layer
302(2).
[0182] Note that in terms of light extraction efficiency, the
charge-generation layer 305 preferably has a light-transmitting
property with respect to visible light (specifically, the
charge-generation layer 305 has a visible light transmittance of
40% or more). Further, the charge-generation layer 305 functions
even if it has lower conductivity than the first electrode 301 or
the second electrode 304.
[0183] The charge-generation layer 305 may have either a structure
in which an electron acceptor (acceptor) is added to an organic
compound having a high hole-transport property or a structure in
which an electron donor (donor) is added to an organic compound
having a high electron-transport property. Alternatively, both of
these structures may be stacked.
[0184] In the case of the structure in which an electron acceptor
is added to an organic compound having a high hole-transport
property, as the organic compound having a high hole-transport
property, for example, an aromatic amine compound such as NPB, TPD,
TDATA, MTDATA, or
4,4'-bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl
(abbreviation: BSPB), or the like can be used. The substances given
here are mainly ones having a hole mobility of 10.sup.-6
cm.sup.2/Vs or higher. Note that any other substance may be used as
long as the substance has a hole-transport property higher than an
electron-transport property.
[0185] Further, as the electron acceptor,
7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:
F.sub.4-TCNQ), chloranil, and the like can be given. In addition, a
transition metal oxide can be given. In addition, an oxide of
metals that belong to Group 4 to Group 8 of the periodic table can
be given. Specifically, vanadium oxide, niobium oxide, tantalum
oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese
oxide, and rhenium oxide are preferable because of their high
electron-accepting properties. Among these metal oxides, molybdenum
oxide is especially preferable since it is stable in the air, has a
low hygroscopic property, and is easily handled.
[0186] On the other hand, in the case of the structure in which an
electron donor is added to an organic compound having a high
electron-transport property, as the organic compound having a high
electron-transport property, for example, a metal complex having a
quinoline skeleton or a benzoquinoline skeleton, such as Alq,
Almq.sub.3, BeBq.sub.2, or BAlq, or the like can be used.
Alternatively, a metal complex having an oxazole-based ligand or a
thiazole-based ligand, such as Zn(BOX).sub.2 or Zn(BTZ).sub.2 can
be used. Other than metal complexes, PBD, OXD-7, TAZ, BPhen, BCP,
or the like can be used. The substances given here are mainly ones
having an electron mobility of 10.sup.-6 cm.sup.2/Vs or higher. An
organic compound having a pyrimidine skeleton may also be used.
Note that any other substance may be used as long as the substance
has an electron-transport property higher than a hole-transport
property.
[0187] As the electron donor, an alkali metal, an alkaline earth
metal, a rare earth metal, a metal belonging to Group 2 or Group 13
of the periodic table, or an oxide or carbonate thereof can be
used. Specifically, lithium (Li), cesium (Cs), magnesium (Mg),
calcium (Ca), ytterbium (Yb), indium (In), lithium oxide, cesium
carbonate, or the like is preferably used. An organic compound such
as tetrathianaphthacene may also be used as the electron donor.
[0188] Note that forming the charge-generation layer 305 by using
any of the above materials can suppress an increase in driving
voltage caused by the stack of the EL layers.
[0189] Although the light-emitting element having two EL layers is
illustrated in FIG. 3A, the present invention can be similarly
applied to a light-emitting element in which n EL layers (n is
three or more) are stacked as illustrated in FIG. 3B. In the case
where a plurality of EL layers are provided between a pair of
electrodes as in the light-emitting element of this embodiment, by
providing a charge-generation layer between the EL layers, the
light-emitting element can emit light in a high luminance region
while the current density is kept low. Since the current density
can be kept low, the element can have a long lifetime. When the
light-emitting element is applied to lighting, voltage drop due to
resistance of an electrode material can be reduced, thereby
achieving homogeneous light emission in a large area. Moreover, a
light-emitting device which can be driven at low voltage and has
low power consumption can be achieved.
[0190] By making the EL layers emit light of different colors from
each other, the light-emitting element can provide light emission
of a desired color as a whole. For example, by forming a
light-emitting element having two EL layers such that the emission
color of the first EL layer and the emission color of the second EL
layer are complementary colors, the light-emitting element can
provide white light emission as a whole. Note that "complementary
colors" refer to colors which produce an achromatic color when
mixed. In other words, when lights obtained from substances which
emit light of complementary colors are mixed, white emission can be
obtained.
[0191] Further, the same can be applied to a light-emitting element
having three EL layers. For example, the light-emitting element as
a whole can provide white light emission when the emission color of
the first EL layer is red, the emission color of the second EL
layer is green, and the emission color of the third EL layer is
blue.
[0192] Note that the structure described in this embodiment can be
combined as appropriate with any of the structures described in the
other embodiments.
Embodiment 4
[0193] In this embodiment, a light-emitting device which includes a
light-emitting element in which a phosphorescent compound and an
organic compound of one embodiment of the present invention are
contained in a light-emitting layer will be described with
reference to FIG. 4.
[0194] A light-emitting device described in this embodiment has a
micro optical resonator (microcavity) structure in which a light
resonant effect between a pair of electrodes is utilized. The
light-emitting device includes a plurality of light-emitting
elements each of which includes at least an EL layer 455 between a
pair of electrodes (a reflective electrode 451 and a
semi-transmissive and semi-reflective electrode 452) as illustrated
in FIG. 4. Further, the EL layer 455 includes at least a first
light-emitting layer 454R, a second light-emitting layer 454G, and
a third light-emitting layer 454B, each of which serves as a
light-emitting region. The EL layer 455 may further include a
hole-injection layer, a hole-transport layer, an electron-transport
layer, an electron-injection layer, a charge generating layer, and
the like. Note that a phosphorescent compound and an organic
compound of one embodiment of the present invention are contained
in at least one of the first light-emitting layer 454R, the second
light-emitting layer 454G and the third light-emitting layer
454B.
[0195] In this embodiment, a light-emitting device which includes
light-emitting elements (a first light-emitting element 450R, a
second light-emitting element 450G and a third light-emitting
element 450B) having different structures as illustrated in FIG. 4
is described.
[0196] The first light-emitting element 450R has a structure in
which a first transparent conductive layer 453a, the EL layer 455,
the semi-transmissive and semi-reflective electrode 452 are
sequentially stacked over the reflective electrode 451. The second
light-emitting element 450G has a structure in which a second
transparent conductive layer 453b, the EL layer 455, and the
semi-transmissive and semi-reflective electrode 452 are
sequentially stacked over the reflective electrode 451. The third
light-emitting element 450B has a structure in which the EL layer
455 and the semi-transmissive and semi-reflective electrode 452 are
sequentially stacked over the reflective electrode 451.
[0197] Note that the reflective electrode 451, the EL layer 455,
and the semi-transmissive and semi-reflective electrode 452 are
common to the light-emitting elements (the first light-emitting
element 450R, the second light-emitting element 450G, and the third
light-emitting element 450B).
[0198] The EL layer 455 includes the first light-emitting layer
454R, the second light-emitting layer 454G, and the third
light-emitting layer 454B. The first light-emitting layer 454R, the
second light-emitting layer 454G, and the third light-emitting
layer 454B emit a light (.lamda..sub.R) having a peak in a
wavelength range from 600 nm to 760 nm, a light (.lamda..sub.G)
having a peak in a wavelength range from 500 nm to 550 nm, and a
light (.lamda..sub.B) having a peak in a wavelength range from 420
nm to 480 nm, respectively. Thus, in each of the light-emitting
elements (the first light-emitting element 450R, the second
light-emitting element 450G and the third light-emitting element
450B), the lights emitted from the first light-emitting layer 454R,
the second light-emitting layer 454G, and the third light-emitting
layer 454B overlap with each other; accordingly, light having a
broad emission spectrum that covers a visible light range can be
emitted. Note that the above wavelengths satisfy the relation of
.lamda..sub.B<.lamda..sub.G<.lamda..sub.R.
[0199] Each of the light-emitting elements described in this
embodiment has a structure in which the EL layer 455 is interposed
between the reflective electrode 451 and the semi-transmissive and
semi-reflective electrode 452. The lights emitted in all directions
from the light-emitting layers included in the EL layer 455 are
resonated by the reflective electrode 451 and the semi-transmissive
and semi-reflective electrode 452 which function as a micro optical
resonator (microcavity). Note that the reflective electrode 451 is
formed using a conductive material having reflectivity, and a film
whose visible light reflectivity is 40% to 100%, preferably 70% to
100%, and whose resistivity is 1.times.10.sup.-2 .OMEGA.cm or lower
is used. In addition, the semi-transmissive and semi-reflective
electrode 452 is formed using a conductive material having
reflectivity and a conductive material having a light-transmitting
property, and a film whose visible light reflectivity is 20% to
80%, preferably 40% to 70%, and whose resistivity is
1.times.10.sup.-2 .OMEGA.cm or lower is used.
[0200] In this embodiment, the thicknesses of the transparent
conductive layers (the first transparent conductive layer 453a and
the second transparent conductive layer 453b) provided in the first
light-emitting element 450R and the second light-emitting element
4500 respectively, are varied between the light-emitting elements,
whereby the light-emitting elements differ in the optical path
length from the reflective electrode 451 to the semi-transmissive
and semi-reflective electrode 452. In other words, in light having
a broad emission spectrum, which is emitted from the light-emitting
layers of each of the light-emitting elements, light with a
wavelength that is resonated between the reflective electrode 451
and the semi-transmissive and semi-reflective electrode 452 can be
enhanced while light with a wavelength that is not resonated
therebetween can be attenuated. Thus, when the elements differ in
the optical path length from the reflective electrode 451 to the
semi-transmissive and semi-reflective electrode 452, light with
different wavelengths can be extracted.
[0201] Note that the optical path length (also referred to as
optical distance) is expressed as a product of an actual distance
and a refractive index, and in this embodiment, is a product of an
actual thickness and n (refractive index). That is, the following
relation is satisfied: optical path length=actual
thickness.times.n.
[0202] Further, the optical path length from the reflective
electrode 451 to the semi-transmissive and semi-reflective
electrode 452 is set to m.lamda..sub.2 (m is a natural number of 1
or more) in the first light-emitting element 450R; the optical path
length from the reflective electrode 451 to the semi-transmissive
and semi-reflective electrode 452 is set to m.lamda..sub.G/2 (m is
a natural number of 1 or more) in the second light-emitting element
450G; and the optical path length from the reflective electrode 451
to the semi-transmissive and semi-reflective electrode 452 is set
to m.lamda..sub.B/2 (m is a natural number of 1 or more) in the
third light-emitting element 450B.
[0203] In this manner, the light (.lamda..sub.R) emitted from the
first light-emitting layer 454R included in the EL layer 455 is
mainly extracted from the first light-emitting element 450R, the
light (.lamda..sub.G) emitted from the second light-emitting layer
454G included in the EL layer 455 is mainly extracted from the
second light-emitting element 450G, and the light (.lamda..sub.B)
emitted from the third light-emitting layer 454B included in the EL
layer 455 is mainly extracted from the third light-emitting element
450B. Note that the light extracted from each of the light-emitting
elements is emitted through the semi-transmissive and
semi-reflective electrode 452 side.
[0204] Further, strictly speaking, the optical path length from the
reflective electrode 451 to the semi-transmissive and
semi-reflective electrode 452 can be the distance from a reflection
region in the reflective electrode 451 to a reflection region in
the semi-transmissive and semi-reflective electrode 452. However,
it is difficult to precisely determine the positions of the
reflection regions in the reflective electrode 451 and the
semi-transmissive and semi-reflective electrode 452; therefore, it
is assumed that the above effect can be sufficiently obtained
wherever the reflection regions may be set in the reflective
electrode 451 and the semi-transmissive and semi-reflective
electrode 452.
[0205] Next, the optical path length from the reflective electrode
451 to the first light-emitting layer 454R is adjusted to
(2n.sub.R-1).lamda..sub.R/4 (n.sub.R is a natural number of 1 or
more) because in the first light-emitting element 450R, light
(first reflected light) that is reflected by the reflective
electrode 451 of the light emitted from the first light-emitting
layer 454R interferes with light (first incident light) that
directly enters the semi-transmissive and semi-reflective electrode
452 from the first light-emitting layer 454R. By adjusting the
optical path length, the phases of the first reflected light and
the first incident light can be aligned with each other and the
light emitted from the first light-emitting layer 454R can be
amplified.
[0206] Note that, strictly speaking, the optical path length from
the reflective electrode 451 to the first light-emitting layer 454R
can be the optical path length from a reflection region in the
reflective electrode 451 to a light-emitting region in the first
light-emitting layer 454R. However, it is difficult to precisely
determine the positions of the reflection region in the reflective
electrode 451 and the light-emitting region in the first
light-emitting layer 454R; therefore, it is assumed that the above
effect can be sufficiently obtained wherever the reflection region
and the light-emitting region may be set in the reflective
electrode 451 and the first light-emitting layer 454R,
respectively.
[0207] Next, the optical path length from the reflective electrode
451 to the second light-emitting layer 454G is adjusted to
(2n.sub.G-1).lamda..sub.G/4 (n.sub.G is a natural number of 1 or
more) because in the second light-emitting element 450G light
(second reflected light) that is reflected by the reflective
electrode 451 of the light emitted from the second light-emitting
layer 454G interferes with light (second incident light) that
directly enters the semi-transmissive and semi-reflective electrode
452 from the second light-emitting layer 454G By adjusting the
optical path length, the phases of the second reflected light and
the second incident light can be aligned with each other and the
light emitted from the second light-emitting layer 454G can be
amplified.
[0208] Note that, strictly speaking, the optical path length from
the reflective electrode 451 to the second light-emitting layer
454G can be the optical path length from a reflection region in the
reflective electrode 451 to a light-emitting region in the second
light-emitting layer 454G However, it is difficult to precisely
determine the positions of the reflection region in the reflective
electrode 451 and the light-emitting region in the second
light-emitting layer 454G; therefore, it is assumed that the above
effect can be sufficiently obtained wherever the reflection region
and the light-emitting region may be set in the reflective
electrode 451 and the second light-emitting layer 454G,
respectively.
[0209] Next, the optical path length from the reflective electrode
451 to the third light-emitting layer 454B is adjusted to
(2n.sub.B-1).lamda..sub.B/4 (n.sub.B is a natural number of 1 or
more) because in the third light-emitting element 450B, light
(third reflected light) that is reflected by the reflective
electrode 451 of the light emitted from the third light-emitting
layer 454B interferes with light (third incident light) that
directly enters the semi-transmissive and semi-reflective electrode
452 from the third light-emitting layer 454B. By adjusting the
optical path length, the phases of the third reflected light and
the third incident light can be aligned with each other and the
light emitted from the third light-emitting layer 454B can be
amplified.
[0210] Note that, strictly speaking, the optical path length from
the reflective electrode 451 to the third light-emitting layer 454B
can be the optical path length from a reflection region in the
reflective electrode 451 to a light-emitting region in the third
light-emitting layer 454B. However, it is difficult to precisely
determine the positions of the reflection region in the reflective
electrode 451 and the light-emitting region in the third
light-emitting layer 454B; therefore, it is assumed that the above
effect can be sufficiently obtained wherever the reflection region
and the light-emitting region may be set in the reflective
electrode 451 and the third light-emitting layer 454B,
respectively.
[0211] Note that although each of the light-emitting elements in
the above-described structure includes a plurality of
light-emitting layers in the EL layer, the present invention is not
limited thereto; for example, the structure of the tandem (stacked
type) light-emitting element which is described in Embodiment 3 can
be combined, in which case a plurality of EL layers is provided so
that a charge generating layer is interposed therebetween in one
light-emitting element and one or more light-emitting layers are
formed in each of the EL layers.
[0212] The light-emitting device described in this embodiment has a
microcavity structure, in which light with wavelengths which differ
depending on the light-emitting elements can be extracted even when
they include the same EL layers, so that it is not necessary to
form light-emitting elements for the colors of R, G, and B.
Therefore, the above structure is advantageous for full color
display owing to easiness in achieving higher resolution display or
the like. In addition, emission intensity with a predetermined
wavelength in the front direction can be increased, whereby power
consumption can be reduced. The above structure is particularly
useful in the case of being applied to a color display (image
display device) including pixels of three or more colors but may
also be applied to lighting or the like.
[0213] Note that the structure described in this embodiment can be
combined as appropriate with any of the structures described in the
other embodiments.
Embodiment 5
[0214] In this embodiment, a light-emitting device which includes a
light-emitting element including a phosphorescent compound and an
organic compound of one embodiment of the present invention will be
described with reference to FIGS. 5A and 5B.
[0215] The light-emitting device including the light-emitting
element of one embodiment of the present invention can be either a
passive matrix light-emitting device or an active matrix
light-emitting device. Note that any of the light-emitting elements
described in the other embodiments can be applied to the
light-emitting device described in this embodiment.
[0216] In this embodiment, as a light-emitting device including the
light-emitting element according to one embodiment of the present
invention, an active matrix light-emitting device is described with
reference to FIGS. 5A and 5B.
[0217] Note that FIG. 5A is a top view illustrating a
light-emitting device and FIG. 5B is a cross-sectional view taken
along A-A' in FIG. 5A. The active matrix light-emitting device
according to this embodiment includes a pixel portion 502 provided
over an element substrate 501, a driver circuit portion (a source
line driver circuit) 503, and a driver circuit portion (a gate line
driver circuit) 504. The pixel portion 502, the driver circuit
portion 503, and the driver circuit portion 504 are sealed with a
sealant 505 between the element substrate 501 and a sealing
substrate 506.
[0218] In addition, over the element substrate 501, a lead wiring
507 for connecting an external input terminal, through which a
signal (e.g., a video signal, a clock signal, a start signal, a
reset signal, or the like) or electric potential from the outside
is transmitted to the driver circuit portion 503 and the driver
circuit portion 504, is provided. Here, an example in which an FPC
508 is provided as the external input terminal is described.
Although only the FPC 508 is illustrated here, a printed wiring
board (PWB) may be attached to the FPC 508. The light-emitting
device in this specification includes, in its category, not only
the light-emitting device itself but also the light-emitting device
provided with the FPC or the PWB.
[0219] Next, a cross-sectional structure is described with
reference to FIG. 5B. The driver circuit portion and the pixel
portion are formed over the element substrate 501, and illustrated
the driver circuit portion 503 which is the source line driver
circuit and the pixel portion 502.
[0220] An example in which a CMOS circuit which is a combination of
an n-channel TFT 509 and a p-channel TFT 510 is formed as the
driver circuit portion 503 is illustrated. Note that a circuit
included in the driver circuit portion may be formed using any of
various circuits, such as a CMOS circuit, a PMOS circuit, or an
NMOS circuit. In this embodiment, although a driver-integrated type
structure in which a driver circuit is formed over a substrate is
described, a driver circuit is not necessarily formed over a
substrate but can be formed outside a substrate.
[0221] The pixel portion 502 is formed of a plurality of pixels
each of which includes a switching TFT 511, a current control TFT
512, and a first electrode 513 which is electrically connected to a
wiring (a source electrode or a drain electrode) of the current
control TFT 512. An insulator 514 is formed so as to cover an edge
portion of the first electrode 513. In this embodiment, the
insulator 514 is formed using a positive photosensitive acrylic
resin. Note that the first electrode 513 is used as an anode and a
second electrode 516 is used as a cathode in this embodiment.
[0222] In addition, in order to obtain favorable coverage with a
film which is to be stacked over the insulator 514, the insulator
514 is preferably formed so as to have a curved surface with
curvature at an upper edge portion or a lower edge portion. For
example, in the case of using a positive photosensitive acrylic
resin as a material for the insulator 514, the insulator 514 is
preferably formed so as to have a curved surface with a curvature
radius (0.2 .mu.m to 3 .mu.m) at the upper edge portion. Note that
the insulator 514 can be formed using either a negative
photosensitive resin or a positive photosensitive resin. It is
possible to use, without limitation to an organic compound, either
an organic compound or an inorganic compound such as silicon oxide
or silicon oxynitride.
[0223] An EL layer 515 and a second electrode 516 are stacked over
the first electrode 513. In the EL layer 515, at least a
light-emitting layer is provided, and the light-emitting layer
contains a phosphorescent compound and an organic compound of one
embodiment of the present invention. Further, in the EL layer 515,
a hole-injection layer, a hole-transport layer, an
electron-transport layer, an electron-injection layer, a
charge-generation layer, and the like can be provided as
appropriate in addition to the light-emitting layer.
[0224] A light-emitting element 517 is formed of a stacked
structure of the first electrode 513, the EL layer 515, and the
second electrode 516. For the first electrode 513, the EL layer
515, and the second electrode 516, the materials described in
Embodiment 1 can be used. Although not illustrated, the second
electrode 516 is electrically connected to an FPC 508 which is an
external input terminal.
[0225] In addition, although the cross-sectional view of FIG. 5B
illustrates only one light-emitting element 517, a plurality of
light-emitting elements are arranged in matrix in the pixel portion
502. Light-emitting elements that emit light of three kinds of
colors (R, G, and B) are selectively formed in the pixel portion
502, whereby a light-emitting device capable of full color display
can be obtained. Alternatively, a light-emitting device which is
capable of full color display may be manufactured by a combination
with color filters.
[0226] Further, the sealing substrate 506 is attached to the
element substrate 501 with the sealant 505, whereby a
light-emitting element 517 is provided in a space 518 surrounded by
the element substrate 501, the sealing substrate 506, and the
sealant 505. Note that the space 518 may be filled with an inert
gas (such as nitrogen and argon) or the sealant 505.
[0227] An epoxy-based resin is preferably used for the sealant 505.
Such a material preferably allows as little moisture and oxygen as
possible to penetrate. As the sealing substrate 506, a plastic
substrate formed of fiberglass-reinforced plastics (FRP), polyvinyl
fluoride (PVF), polyester, acrylic resin, or the like can be used
besides a glass substrate or a quartz substrate.
[0228] As described above, an active matrix light-emitting device
utilizing phosphorescence can be obtained.
[0229] Note that the structure described in this embodiment can be
combined as appropriate with any of the structures described in the
other embodiments.
Embodiment 6
[0230] In this embodiment, an electronic device which partly
includes the light-emitting device of one embodiment of the present
invention which is described in the above embodiments will be
described. Examples of the electronic device include cameras such
as video cameras and digital cameras, goggle type displays,
navigation systems, audio replay devices (e.g., car audio systems
and audio systems), computers, game machines, portable information
terminals (e.g., mobile computers, mobile phones, smartphones,
portable game machines, e-book readers, and tablet terminals), and
image replay devices in which a recording medium is provided
(specifically, devices that are capable of replaying recording
media such as digital versatile discs (DVDs) and equipped with a
display device that can display an image). Specific examples of
these electronic devices will be described with reference to FIGS.
6A to 6D and FIGS. 7A-1 to 7B.
[0231] FIG. 6A illustrates a television set according to one
embodiment of the present invention, which includes a housing 611,
a supporting base 612, a display portion 613, speaker portions 614,
video input terminals 615, and the like. In this television set,
the light-emitting device of one embodiment of the present
invention can be applied to the display portion 613. Since the
light-emitting device of one embodiment of the present invention is
driven at low voltage and has high current efficiency, by the
application of the light-emitting device of one embodiment of the
present invention, a television set with reduced power consumption
can be obtained.
[0232] FIG. 6B illustrates a computer according to one embodiment
of the present invention, which includes a main body 621, a housing
622, a display portion 623, a keyboard 624, an external connection
port 625, a pointing device 626, and the like. In this computer,
the light-emitting device of one embodiment of the present
invention can be applied to the display portion 623. Since the
light-emitting device of one embodiment of the present invention is
driven at low voltage and has high current efficiency, by the
application of the light-emitting device of one embodiment of the
present invention, a computer with reduced power consumption can be
obtained.
[0233] FIG. 6C illustrates a mobile phone according to one
embodiment of the present invention, which includes a main body
631, a housing 632, a display portion 633, an audio input portion
634, an audio output portion 635, operation keys 636, an external
connection port 637, an antenna 638, and the like. In this mobile
phone, the light-emitting device of one embodiment of the present
invention can be applied to the display portion 633. Since the
light-emitting device of one embodiment of the present invention is
driven at low voltage and has high current efficiency, by the
application of the light-emitting device of one embodiment of the
present invention, a mobile phone with reduced power consumption
can be obtained.
[0234] FIG. 6D illustrates a camera according to one embodiment of
the present invention, which includes a main body 641, a display
portion 642, a housing 643, an external connection port 644, a
remote control receiving portion 645, an image receiving portion
646, a battery 647, an audio input portion 648, operation keys 649,
an eyepiece portion 650, and the like. In this camera, the
light-emitting device of one embodiment of the present invention
can be applied to the display portion 642. Since the light-emitting
device of one embodiment of the present invention is driven at low
voltage and has high current efficiency, by the application of the
light-emitting device of one embodiment of the present invention, a
camera with reduced power consumption can be obtained.
[0235] FIGS. 7A-1 to 7B illustrate examples of tablet terminals
according to one embodiment of the present invention. FIGS. 7A-1,
7A-2, and 7A-3 illustrate a tablet PC 5000, and FIG. 7B illustrates
a tablet PC 6000.
[0236] FIGS. 7A-1, 7A-2, and 7A-3 are a front view, a side view,
and a rear view of the tablet PC 5000, respectively. FIG. 7B is a
front view of the tablet PC 6000.
[0237] The tablet PC 5000 includes a housing 5001, a display
portion 5003, a power button 5005, a front camera 5007, a rear
camera 5009, a first external connection terminal 5011, a second
external connection terminal 5013, and the like.
[0238] In addition, the display portion 5003 is incorporated in the
housing 5001 and can be used as a touch panel. For example,
e-mailing or schedule management can be performed by touching an
icon 5015 and the like on the display portion 5003. Further, the
front camera 5007 is incorporated on the front side of the housing
5001, whereby an image on the user's side can be taken. The rear
camera 5009 is incorporated in the rear side of the housing 5001,
whereby an image on the opposite side of the user can be taken.
Further, the housing 5001 includes the first external connection
terminal 5011 and the second external connection terminal 5013. For
example, sound can be output to an earphone or the like through the
first external connection terminal 5011, and data can be moved
through the second external connection terminal 5013.
[0239] The tablet PC 6000 in FIG. 7B includes a first housing 6001,
a second housing 6003, a hinge portion 6005, a first display
portion 6007, a second display portion 6009, a power button 6011, a
first camera 6013, a second camera 6015, and the like.
[0240] The first display portion 6007 is incorporated in the first
housing 6001. The second display portion 6009 is incorporated in
the second housing 6003. For example, the first display portion
6007 and the second display portion 6009 are used as a display
panel and a touch panel, respectively. A user can select images,
enter characters, and so on by touching an icon 6019 displayed on
the second display portion 6009 or a keyboard 6021 (actually, a
keyboard image displayed on the second display portion 6009) while
looking at a text icon 6017 displayed on the first display portion
6007. Alternatively, the first display portion 6007 and the second
display portion 6009 may be a touch panel and a display panel,
respectively; the first display portion 6007 and the second display
portion 6009 may be touch panels.
[0241] The first housing 6001 and the second housing 6003 are
connected to each other and open and close on the hinge portion
6005. In such a structure, the first display portion 6007
incorporated in the first housing 6001 and the second display
portion 6009 incorporated in the second housing 6003 are preferably
made to face each other, in which case the surfaces of the first
display portion 6007 and the second display portion 6009 (e.g.,
plastic substrates) can be protected.
[0242] Alternatively, the first housing 6001 and the second housing
6003 may be separated by the hinge portion 6005 (so-called
convertible type). Thus, the application range of the tablet PC
6000 can be extended: for example, the first housing 6001 is used
in a vertical orientation and the second housing 6003 is used in a
horizontal orientation.
[0243] Further, the first camera 6013 and the second camera 6015
can take 3D images.
[0244] The tablet PC 5000 and the tablet PC 6000 may send and
receive data wirelessly. For example, through wireless internet
connection, desired data can be purchased and downloaded.
[0245] The tablet PCs 5000 and 6000 can have other functions such
as a function of displaying various kinds of data (e.g., a still
image, a moving image, and a text image), a function of displaying
a calendar, a date, the time, or the like on the display portion, a
touch-input function of operating or editing the data displayed on
the display portion by touch input, and a function of controlling
processing by various kinds of software (programs). A detector such
as a photodetector capable of optimizing display luminance in
accordance with the amount of outside light or a sensor for
detecting inclination, like a gyroscope or an acceleration sensor,
can be included.
[0246] The light-emitting device of one embodiment of the present
invention can be applied to the display portion 5003 of the tablet
PC 5000, the first display portion 6007 of the tablet PC 6000,
and/or the second display portion 6009 of the tablet PC 6000. Since
the light-emitting device of one embodiment of the present
invention is driven at low voltage and has high current efficiency,
by the application of the light-emitting device of one embodiment
of the present invention, a tablet terminal with reduced power
consumption can be obtained.
[0247] As described above, the applicable range of the
light-emitting device of one embodiment of the present invention is
so wide that the light-emitting device can be applied to electronic
devices in a variety of fields. With the use of the light-emitting
device of one embodiment of the present invention, an electronic
device with reduced power consumption can be obtained.
[0248] The light-emitting device of one embodiment of the present
invention can also be used as a lighting device. Specific examples
of the lighting device are described with reference to FIGS. 8A to
8C.
[0249] FIG. 8A illustrates an example of a liquid crystal display
device using the light-emitting device of one embodiment of the
present invention as a backlight. The liquid crystal display device
illustrated in FIG. 8A includes a housing 701, a liquid crystal
layer 702, a backlight 703, and a housing 704. The liquid crystal
layer 702 is connected to a driver IC 705. The light-emitting
device of one embodiment of the present invention is used as the
backlight 703, and current is supplied through a terminal 706. By
using the light-emitting device of one embodiment of the present
invention as a backlight of a liquid crystal display device as
described above, a backlight with low power consumption can be
obtained. Moreover, since the light-emitting device of one
embodiment of the present invention is a lighting device for
surface light emission and the enlargement of the light-emitting
device is possible, the backlight can be made larger. Thus, a
larger-area liquid crystal display device with low power
consumption can be obtained.
[0250] Next, FIG. 8B illustrates an example in which the
light-emitting device of one embodiment of the present invention is
used for a desk lamp which is a lighting device. The desk lamp
illustrated in FIG. 8B includes a housing 801 and a light source
802, and the light-emitting device of one embodiment of the present
invention is used as the light source 802. Since the light-emitting
device of one embodiment of the present invention is driven at low
voltage and has high current efficiency, by the application of the
light-emitting device of one embodiment of the present invention, a
desk lamp with reduced power consumption can be obtained.
[0251] FIG. 8C illustrates an example in which the light-emitting
device of one embodiment of the present invention is used for an
indoor lighting device 901. Since the light-emitting device of an
embodiment of the present invention can also have a larger area,
the light-emitting device of an embodiment of the present invention
can be used as a lighting system having a large area. Since the
light-emitting device of one embodiment of the present invention is
driven at low voltage and has high current efficiency, by the
application of the light-emitting device of one embodiment of the
present invention, a lighting device with reduced power consumption
can be obtained. In a room where the light-emitting device of one
embodiment of the present invention is used for the indoor lighting
device 901 as described above, a television set 902 of one
embodiment of the present invention as described with reference to
FIG. 6A can be installed so that public broadcasting and movies can
be watched.
[0252] Note that this embodiment can be combined with any of the
other embodiments as appropriate.
Example 1
[0253] In this example, a synthesis method of
4-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylam-
ine (abbreviation: PCBAPDBq), which is represented by Structural
Formula (100) in Embodiment 1, as an organic compound of one
embodiment of the present invention will be described.
##STR00056##
Synthesis of
4-(Dibenzo[f,h]quinoxalin-2-yl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylam-
ine (abbreviation: PCBAPDBq)
[0254] A synthesis scheme of
4-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylam-
ine (abbreviation: PCBAPDBq) is shown in (B-1).
##STR00057##
[0255] In a 50 mL three-neck flask were put 0.23 g (0.87 mmol) of
2-chlorodibenzo[f,h]quinoxaline, 0.50 g (0.94 mmol) of
4-{N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-N-phenylamino}phenylboronic
acid, 5 mL of toluene, 1 mL of ethanol, and 1.5 mL of a 2 M aqueous
solution of potassium carbonate. This mixture was degassed by being
stirred under reduced pressure, and the air in the flask was
replaced with nitrogen. To this mixture was added 20 mg (17
.mu.mol) of tetrakis(triphenylphosphine)palladium(0). This mixture
was stirred at 80.degree. C. under a nitrogen stream for 20 hours.
After predetermined time passed, water and toluene were added to
this mixture to extract the obtained aqueous layer with toluene.
The solution of the obtained extract and the organic layer were
combined and washed with a saturated aqueous solution of sodium
hydrogen carbonate and saturated saline, and the organic layer was
dried over magnesium sulfate. The obtained mixture was
gravity-filtered to give filtrate, and the filtrate was
concentrated to give a solid. The obtained solid was purified by
silica gel column chromatography (toluene: hexane=1:1) and
recrystallization from toluene and methanol was performed to give
the target substance as 0.53 g of yellow powder in a yield of
85%.
[0256] By a train sublimation method, 0.52 g of the obtained powder
of
4-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylam-
ine was purified. In the purification,
4-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylam-
ine was heated at 320.degree. C. under the conditions that the
pressure was 4.2 Pa and the argon flow rate was 5.0 mL/min. After
the purification, 0.43 g of yellow powder of
4-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylam-
ine was obtained at a collection rate of 83%.
[0257] A nuclear magnetic resonance spectrometry (.sup.1H NMR)
identified this compound as
4-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylam-
ine (abbreviation: PCBAPDBq) which was the target substance.
[0258] .sup.1H NMR data of the obtained substance are as follows:
.sup.1H NMR (CDCl.sub.3, 300 MHz): .delta.=7.14 (t, J=7.2 Hz, 1H),
7.29-7.83 (m, 24H), 8.20 (d, J=7.8 Hz, 1H), 8.27 (d, J=8.7 Hz, 2H),
8.37 (s, 1H), 8.66 (d, J=8.1 Hz, 2H), 9.20-9.24 (m, 1H), 9.36 (s,
1H), 9.40 (d, J=7.2 Hz, 1H).
[0259] FIGS. 9A and 9B are .sup.1H-NMR charts. Note that FIG. 9B is
a chart where the range of from 7.0 ppm to 9.5 ppm in FIG. 9A is
enlarged.
[0260] FIGS. 10A and 10B show an absorption spectrum and an
emission spectrum, respectively, of PCBAPDBq in a toluene solution
of PCBAPDBq. FIGS. 11A and 11B show an absorption spectrum and an
emission spectrum, respectively, of a thin film of PCBAPDBq. The
absorption spectrum was measured with a UV-visible
spectrophotometer (V550, produced by JASCO Corporation). The
measurements were performed with samples prepared in such a manner
that the toluene solution was put in a quartz cell and the thin
film was obtained by deposition of PCBAPDBq on a quartz substrate
by evaporation. The absorption spectrum of PCBAPDBq in the toluene
solution of PCBAPDBq was obtained by subtracting the absorption
spectra of quartz and toluene from those of quartz and the toluene
solution, and the absorption spectrum of the thin film of PCBAPDBq
was obtained by subtracting the absorption spectrum of the quartz
substrate from those of the quartz substrate and the thin film.
[0261] In each of FIGS. 10A, 10B, 11A, and 11B, the horizontal axis
represents wavelength (nm) and the vertical axis represents
intensity (arbitrary unit). In the case of the toluene solution,
the absorption peaks are around 282 nm, 325 nm, and 416 nm, and the
emission wavelength peak is around 483 nm. In the case of the thin
film, the absorption peaks are around 254 nm, 333 nm, and 425 nm,
and the emission wavelength peak is around 522 nm.
Example 2
[0262] In this example, a synthesis method of
3-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9H-carbazol-9-yl)triphenylamine
(abbreviation: mYGAPDBq), which is an organic compound of one
embodiment of the present invention and represented by Structural
Formula (201) in Embodiment 1, will be described.
##STR00058##
Synthesis of
3-(Dibenzo[f,h]quinoxalin-2-yl)-4'-(9H-carbazol-9-yl)triphenylamine
(abbreviation: mYGAPDBq)
[0263] A synthesis scheme of
3-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9H-carbazol-9-yl)triphenylamine
(abbreviation: mYGAPDBq) is shown in (C-1).
##STR00059##
[0264] In a 200 mL three-neck flask were put 1.1 g (2.9 mmol) of
2-(3-bromophenyl)dibenzo[f,h]quinoxaline, 1.0 g (3.0 mmol) of
9-[4-(N-phenylamino)phenyl]-9H-carbazole, and 0.60 g (6.2 mmol) of
sodium tert-butoxide. The air in the flask was replaced with
nitrogen under reduced pressure. To this mixture was added 30 mL of
xylene, and the mixture was degassed by being stirred under reduced
pressure. To this mixture were added 0.2 mL of
tri(tert-butyl)phosphine (10 wt % hexane solution) and 55 g (96
.mu.mol) of bis(dibenzylideneacetone)palladium(0). This mixture was
refluxed at 140.degree. C. under a nitrogen stream for 7 hours.
After predetermined time passed, water was added to the obtained
mixture to extract an aqueous layer with toluene. The solution of
the obtained extract and the organic layer were combined and washed
with a saturated aqueous solution of sodium carbonate and saturated
saline, and the resulting organic layer was dried with magnesium
sulfate. The obtained mixture was gravity filtered, and the
filtrate was concentrated to give an oily substance. The obtained
oily substance was purified by silica gel column chromatography
(toluene: hexane=1:2) to give an oily substance. Methanol was added
to the obtained oily substance, followed by irradiation with
ultrasonic waves. The precipitated solid was collected by suction
filtration to give the target substance as 0.57 g of yellow powder
in a yield of 30%.
[0265] By a train sublimation method, 0.55 g of the obtained yellow
powder of
2-(3-{N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylamino}phenyl)dibenzo[f,h]-
quinoxaline was purified. In the purification,
2-(3-{N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylamino}phenyl)dibenzo[f,h]qui-
noxaline was heated at 300.degree. C. under the conditions that the
pressure was 3.0 Pa and the argon flow rate was 5.0 mL/min for 16
hours. After the purification, 0.45 g of yellow glass-like
substance of
2-(3-{N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylamino}phenyl)dibenzo[f,h]qui-
noxaline was obtained at a collection rate of 82%.
[0266] A nuclear magnetic resonance spectrometry ('H-NMR)
identified this compound as
3-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9H-carbazol-9-yl)triphenylamine
(abbreviation: mYGAPDBq) which was the target substance.
[0267] .sup.1H NMR data of the obtained substance are as follows:
.sup.1H NMR (DMSO-d.sub.6, 300 MHz): .delta.=7.21-7.51 (m, 14H),
7.60-7.73 (m, 4H), 7.81-7.92 (m, 3H), 8.18 (d, J=7.8 Hz, 1H),
8.25-8.30 (m, 3H), 8.88 (d, J=8.4 Hz, 2H), 9.08 (d, J=7.8 Hz, 1H),
9.15 (d, J=7.8 Hz, 1H), 9.65 (s, 1H).
[0268] FIGS. 26A and 26B are .sup.1H-NMR charts. Note that FIG. 26B
is a chart where the range of from 7.0 ppm to 10.0 ppm in FIG. 26A
is enlarged.
[0269] FIGS. 27A and 27B show an absorption spectrum and an
emission spectrum, respectively, of mYGAPDBq in a toluene solution
of mYGAPDBq. FIGS. 28A and 28B show an absorption spectrum and an
emission spectrum, respectively, of a thin film of mYGAPDBq. The
absorption spectrum was measured with a UV-visible
spectrophotometer (V550, produced by JASCO Corporation). The
measurements were performed with samples prepared in such a manner
that the toluene solution was put in a quartz cell and the thin
film was obtained by deposition of mYGAPDBq on a quartz substrate.
The absorption spectrum of mYGAPDBq in the toluene solution of
mYGAPDBq was obtained by subtracting the absorption spectra of
quartz and toluene from those of quartz and the toluene solution,
and the absorption spectrum of the thin film of mYGAPDBq was
obtained by subtracting the absorption spectrum of the quartz
substrate from those of the quartz substrate and the thin film.
[0270] In each of FIGS. 27A, 27B, 28A, and 28B, the horizontal axis
represents wavelength (nm) and the vertical axis represents
intensity (arbitrary unit). In the case of the toluene solution,
the absorption peaks are around 215 nm, 280 nm, 293 nm, and 372 nm,
and the emission wavelength peak is around 502 nm. In the case of
the thin film, the absorption peaks are around 244 nm, 261 nm, 288
nm, 313 nm, and 377 nm, and the emission wavelength peak is around
500 nm.
[0271] Next, mYGAPDBq obtained in this example was analyzed by
liquid chromatography mass spectrometry (LC/MS).
[0272] The LC/MS analysis was carried out with Acquity UPLC
(produced by Waters Corporation) and Xevo G2 T of MS (produced by
Waters Corporation).
[0273] In the MS analysis, ionization was carried out by an
electrospray ionization (ESI) method. At this time, the capillary
voltage and the sample cone voltage were set to 3.0 kV and 30 V,
respectively, and detection was performed in a positive mode. A
component which underwent the ionization under the above-described
conditions was collided with an argon gas in a collision cell to
dissociate into product ions. Energy (collision energy) for the
collision with argon was 70 eV. The mass range for the measurement
was m/z=100 to 1200.
[0274] FIG. 29 shows results of the MS analysis. The results in
FIG. 29 show that as for mYGAPDBq which is one embodiment of the
present invention, peaks of product ions are detected mainly around
m/z=167, m/z=243, m/z=394, m/z=445, and m/z=472, and a peak derived
from a precursor ion is detected around m/z=639. Here, "around" is
used to indicate changes in values of product ions and precursor
ions due to the existence and absence of hydrogen ions and the
existence of isotopes and to indicate that these changes in values
are in an acceptable range. Note that the results shown in FIG. 29
show characteristics derived from mYGAPDBq and therefore can be
regarded as important data for identifying mYGAPDBq contained in
the mixture.
Example 3
[0275] In this example, a light-emitting element 1 which includes a
light-emitting layer containing a phosphorescent compound and the
organic compound which is described in Embodiment 1 and Example 1
and is represented by Structural Formula (100) was evaluated.
Chemical formulae of materials used in this example are shown
below.
##STR00060## ##STR00061##
[0276] The light-emitting element 1 is described with reference to
FIG. 12. A manufacturing method of the light-emitting element 1 of
this example is described below.
(Light-Emitting Element 1)
[0277] First, an indium oxide-tin oxide compound containing silicon
or silicon oxide (ITO--SiO.sub.2, hereinafter abbreviated to ITSO)
was deposited by a sputtering method over a substrate 1100, whereby
a first electrode 1101 was formed. Note that the composition ratio
of In.sub.2O.sub.3 to SnO.sub.2 and SiO.sub.2 in the target used
was 85:10:5 [wt %]. The thickness of the first electrode 1101 was
110 nm and the electrode area was 2 mm.times.2 mm. Here, the first
electrode 1101 functions as an anode of the light-emitting
element.
[0278] Next, as pretreatment for forming the light-emitting element
over the substrate 1100, the surface of the substrate was washed,
baked at 200.degree. C. for one hour, and subjected to UV ozone
treatment for 370 seconds.
[0279] After that, the substrate 1100 was transferred into a vacuum
evaporation apparatus where the pressure had been reduced to
approximately 10.sup.-4 Pa, and subjected to vacuum baking at
170.degree. C. for 30 minutes in a heating chamber of the vacuum
evaporation apparatus. Then, the substrate 1100 was cooled down for
about 30 minutes.
[0280] Next, the substrate 1100 provided with the first electrode
1101 was fixed to a substrate holder in the vacuum evaporation
apparatus so that a surface on which the first electrode 1101 was
provided faced downward. The pressure in the vacuum evaporation
apparatus was reduced to about 10.sup.-4 Pa. Then,
4,4',4''-(1,3,5-benzenetriyl)tri(dibenzothiophene) (abbreviation:
DBT3P-II) and molybdenum oxide were co-evaporated to form a
hole-injection layer 1111 on the first electrode 1101. The
thickness of the hole-injection layer 1111 was set to 40 nm, and
the weight ratio of DBT3P-II to molybdenum oxide was adjusted to
4:2 (=DBT3P-II: molybdenum oxide). Note that the co-evaporation
method refers to an evaporation method in which evaporation is
carried out from a plurality of evaporation sources at the same
time in one treatment chamber.
[0281] Next, on the hole-injection layer 1111, a film of
4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:
BPAFLP) was formed to a thickness of 20 nm, whereby a
hole-transport layer 1112 was formed.
[0282] Further,
4-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylam-
ine (abbreviation: PCBAPDBq) synthesized in Example 1 and
(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (another
name:
bis[2-(6-phenyl-4-pyrimidinyl-.sub.KN3)phenyl-.sub.KC](2,4-pentanedionato-
-.sub.K.sup.2O,O')iridium(III)) (abbreviation:
[Ir(dppm).sub.2(acac)]) were co-evaporated, whereby a
light-emitting layer 1113 was formed on the hole-transport layer
1112. Here, the weight ratio of PCBAPDBq to [Ir(dppm).sub.2(acac)]
was adjusted to 1:0.05 (=PCBAPDBq: [Ir(dppm).sub.2(acac)]). The
thickness of the light-emitting layer 1113 was set to 40 nm.
[0283] Note that [Ir(dppm).sub.2(acac)] is a phosphorescent
compound and a guest material (dopant) in the light-emitting layer
1113. Further, PCBAPDBq is a host material in the light-emitting
layer 1113.
[0284] Next, on the light-emitting layer 1113, a film of
2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2mDBTPDBq-II) was formed to a thickness of 10 nm,
whereby a first electron-transport layer 1114a was formed.
[0285] Next, a film of bathophenanthroline (abbreviation: BPhen)
was formed to a thickness of 20 nm on the first electron-transport
layer 1114a, whereby a second electron-transport layer 1114b was
formed.
[0286] Further, a film of lithium fluoride (LiF) was formed to a
thickness of 1 nm on the second electron-transport layer 1114b by
evaporation, whereby an electron-injection layer 1115 was
formed.
[0287] Lastly, a film of aluminum was formed to a thickness of 200
nm by evaporation, whereby a second electrode 1103 functioning as a
cathode was formed. Thus, the light-emitting element 1 of this
example was manufactured.
[0288] Table 1 shows an element structure of the light-emitting
element 1 obtained as described above.
TABLE-US-00001 TABLE 1 Hole- Hole- Electron- First injection
transport First electron- Second electron- injection Second
electrode layer layer Light-emitting layer transport layer
transport layer layer electrode Light- ITSO DBT3P- BPAFLP
PCBAPDBq:Ir(dppm).sub.2acac 2mDBTPDBq- II BPhen LiF Al emitting 110
nm II:MoOx 20 nm (=1:0.05) 10 nm 20 nm 1 nm 200 nm element 1 (=4:2)
40 nm 40 nm
[0289] Next, in a glove box containing a nitrogen atmosphere, the
light-emitting element 1 was sealed so as not to be exposed to the
air (a sealant was applied onto an outer edge of the element and
heat treatment was performed at 80.degree. C. for one hour at the
time of sealing). After that, operation characteristics of the
light-emitting element 1 were measured. Note that the measurement
was carried out at room temperature (in an atmosphere kept at
25.degree. C.).
[0290] FIG. 13 shows luminance versus current density
characteristics of the light-emitting element 1. In FIG. 13, the
horizontal axis represents current density (mA/cm.sup.2) and the
vertical axis represents luminance (cd/m.sup.2). FIG. 14 shows
luminance versus voltage characteristics of the light-emitting
element 1. In FIG. 14, the horizontal axis represents voltage (V)
and the vertical axis represents luminance (cd/m.sup.2). FIG. 15
shows current efficiency versus luminance characteristics of the
light-emitting element 1. In FIG. 15, the horizontal axis
represents luminance (cd/m.sup.2) and the vertical axis represents
current efficiency (cd/A). FIG. 16 shows current versus voltage
characteristics of the light-emitting element 1. In FIG. 16, the
horizontal axis represents voltage (V) and the vertical axis
represents current (mA).
[0291] FIG. 13 and FIG. 15 show that the light-emitting element 1
has high efficiency. FIG. 13, FIG. 14, and FIG. 16 show that the
light-emitting element 1 has low driving voltage and low power
consumption.
[0292] Table 2 shows the voltage (V), current density
(mA/cm.sup.2), CIE chromaticity coordinates (x, y), luminance
(cd/m.sup.2), current efficiency (cd/A), and external quantum
efficiency (%) of the light-emitting element 1 at a luminance of
770 cd/m.sup.2.
TABLE-US-00002 TABLE 2 Current Current External Voltage density
Chromaticity Luminance efficiency quantum (V) (mA/cm.sup.2) x y
(cd/m.sup.2) (cd/A) efficiency(%) Light-emitting 2.6 1.5 0.56 0.43
770 53 20 element 1
[0293] FIG. 17 shows an emission spectrum at a current density of
the light-emitting element 1 of 2.5 mA/cm.sup.2. As shown in FIG.
17, the emission spectrum of the light-emitting element 1 has a
peak at 583 nm.
[0294] As shown in Table 2, the CIE chromaticity coordinates of the
light-emitting element 1 at a luminance of 770 cd/m.sup.2 were (x,
y)=(0.56, 0.43). The results show that light emanating from the
dopant was obtained.
[0295] As described above, the light-emitting element 1 in which
the phosphorescent compound and PCBAPDBq which is the organic
compound of one embodiment of the present invention are contained
in the light-emitting layer can efficiently emit light in the
orange wavelength range. It can be said that PCBAPDBq is suitable
as a host material for a material emitting light with a wavelength
equal to or longer than that of orange light.
[0296] Next, a reliability test was performed on the light-emitting
element 1. FIG. 18 shows results of the reliability test.
[0297] In the reliability test, the light-emitting element 1 was
driven under the conditions where the initial luminance was 5000
cd/m.sup.2 and the current density was constant. The results are
shown in FIG. 18. The horizontal axis represents driving time (h)
of the element and the vertical axis represents normalized
luminance (%) on the assumption that the initial luminance is 100%.
According to FIG. 18, it takes about 510 hours for the normalized
luminance of the light-emitting element 1 to decline to 57%.
[0298] FIG. 18 shows that the light-emitting element 1 has a long
lifetime.
[0299] The above results show that the light-emitting element 1
which includes the light-emitting layer containing the
phosphorescent compound and PCBAPDBq which is the organic compound
of one embodiment of the present invention has high efficiency, low
driving voltage, low power consumption, and a long lifetime.
Example 4
[0300] In this example, a light-emitting element 2 which includes a
light-emitting layer containing a phosphorescent compound and the
organic compound which is described in Embodiment 1 and Example 1
and is represented by Structural Formula (100) was evaluated. Note
that the light-emitting layer of the light-emitting element 2
described in this example has a different structure from the
light-emitting layer of the light-emitting element 1 described in
Example 3. Chemical formulae of materials used in this example are
shown below.
##STR00062## ##STR00063##
[0301] The light-emitting element 2 is described with reference to
FIG. 12. A manufacturing method of the light-emitting element 2 of
this example is described below.
(Light-Emitting Element 2)
[0302] First, an indium oxide-tin oxide compound containing silicon
or silicon oxide (ITO--SiO.sub.2, hereinafter abbreviated to ITSO)
was deposited by a sputtering method over a substrate 1100, whereby
a first electrode 1101 was formed. Note that the composition ratio
of In.sub.2O.sub.3 to SnO.sub.2 and SiO.sub.2 in the target used
was 85:10:5 [wt %]. The thickness of the first electrode 1101 was
110 nm and the electrode area was 2 mm.times.2 mm. Here, the first
electrode 1101 functions as an anode of the light-emitting
element.
[0303] Next, as pretreatment for forming the light-emitting element
over the substrate 1100, the surface of the substrate was washed,
baked at 200.degree. C. for one hour, and subjected to UV ozone
treatment for 370 seconds.
[0304] After that, the substrate 1100 was transferred into a vacuum
evaporation apparatus where the pressure had been reduced to
approximately 10.sup.-4 Pa, and subjected to vacuum baking at
170.degree. C. for 30 minutes in a heating chamber of the vacuum
evaporation apparatus. Then, the substrate 1100 was cooled down for
about 30 minutes.
[0305] Next, the substrate 1100 provided with the first electrode
1101 was fixed to a substrate holder in the vacuum evaporation
apparatus so that a surface on which the first electrode 1101 was
provided faced downward. The pressure in the vacuum evaporation
apparatus was reduced to about 10.sup.-4 Pa. Then,
4,4',4''-(1,3,5-benzenetriyl)tri(dibenzothiophene) (abbreviation:
DBT3P-II) and molybdenum oxide were co-evaporated to form a
hole-injection layer 1111 on the first electrode 1101. The
thickness of the hole-injection layer 1111 was set to 40 nm, and
the weight ratio of DBT3P-II to molybdenum oxide was adjusted to
4:2 (=DBT3P-II: molybdenum oxide).
[0306] Next, on the hole-injection layer 1111, a film of
4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:
BPAFLP) was formed to a thickness of 20 nm, whereby a
hole-transport layer 1112 was formed.
[0307] Further,
2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2mDBTPDBq-II),
4-(dibenzo[f,h]quinoxalin-2-yl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylam-
ine (abbreviation: PCBAPDBq) synthesized in Example 1,
(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (another
name:
bis[2-(6-phenyl-4-pyrimidinyl-.sub.KN3)phenyl-.sub.KC](2,4-pentanedionato-
-.sub.K.sup.2O,O')iridium(III)) (abbreviation:
[Ir(dppm).sub.2(acac)]) were co-evaporated, whereby a
light-emitting layer 1113 was formed on the hole-transport layer
1112. Here, the weight ratio of 2mDBTPDBq-II to PCBAPDBq and
[Ir(dppm).sub.2(acac)] was adjusted to 0.6:0.4:0.05 (=2mDBTPDBq-II:
PCBAPDBq: [Ir(dppm).sub.2(acac)]). The thickness of the
light-emitting layer 1113 was set to 40 nm.
[0308] Note that [Ir(dppm).sub.2(acac)] is a phosphorescent
compound and a guest material (dopant) in the light-emitting layer
1113. Further, 2mDBTPDBq-II is a host material in the
light-emitting layer 1113. Further, PCBAPDBq is an assist material
in the light-emitting layer 1113.
[0309] Next, a film of 2mDBTPDBq-II was formed to a thickness of 10
nm on the light-emitting layer 1113, whereby a first
electron-transport layer 1114a was formed.
[0310] Next, a film of bathophenanthroline (abbreviation: BPhen)
was formed to a thickness of 20 nm on the first electron-transport
layer 1114a, whereby a second electron-transport layer 1114b was
formed.
[0311] Further, a film of lithium fluoride (LiF) was formed to a
thickness of 1 nm on the second electron-transport layer 1114b by
evaporation, whereby an electron-injection layer 1115 was
formed.
[0312] Lastly, a film of aluminum was formed to a thickness of 200
nm by evaporation, whereby a second electrode 1103 functioning as a
cathode was formed. Thus, the light-emitting element 2 of this
example was manufactured.
[0313] Table 3 shows an element structure of the light-emitting
element 2 obtained as described above.
TABLE-US-00003 TABLE 3 Hole- Hole- Electron- First injection
transport First electron- Second electron- injection Second
electrode layer layer Light-emitting layer transport layer
transport layer layer electrode Light- ITSO DBT3P- BPAFLP
2mDBTPDBq- 2mDBTPDBq- II BPhen LiF Al emitting 110 nm II:MoOx 20 nm
II:PCBAPDBq:Ir(dppm).sub.2acac 10 nm 20 nm 1 nm 200 nm element 2
(=4:2) (=0.6:0.4:0.05) 40 nm 40 nm
[0314] Next, in a glove box containing a nitrogen atmosphere, the
light-emitting element 2 was sealed so as not to be exposed to the
air (a sealant was applied onto an outer edge of the element and
heat treatment was performed at 80.degree. C. for one hour at the
time of sealing). After that, operation characteristics of the
light-emitting element 2 were measured. Note that the measurement
was carried out at room temperature (in an atmosphere kept at
25.degree. C.).
[0315] FIG. 19 shows luminance versus current density
characteristics of the light-emitting element 2. In FIG. 19, the
horizontal axis represents current density (mA/cm.sup.2) and the
vertical axis represents luminance (cd/m.sup.2). FIG. 20 shows
luminance versus voltage characteristics of the light-emitting
element 2. In FIG. 20, the horizontal axis represents voltage (V)
and the vertical axis represents luminance (cd/m.sup.2). FIG. 21
shows current efficiency versus luminance characteristics of the
light-emitting element 2. In FIG. 21, the horizontal axis
represents luminance (cd/m.sup.2) and the vertical axis represents
current efficiency (cd/A). FIG. 22 shows current versus voltage
characteristics of the light-emitting element 2. In FIG. 22, the
horizontal axis represents voltage (V) and the vertical axis
represents current (mA).
[0316] FIG. 19 and FIG. 21 show that the light-emitting element 2
has high efficiency. FIG. 19, FIG. 20, and FIG. 22 show that the
light-emitting element 2 has low driving voltage and low power
consumption.
[0317] Table 4 shows the voltage (V), current density
(mA/cm.sup.2), CIE chromaticity coordinates (x, y), luminance
(cd/m.sup.2), current efficiency (cd/A), and external quantum
efficiency (%) of the light-emitting element 2 at a luminance of
1097 cd/m.sup.2.
TABLE-US-00004 TABLE 4 Current Current External Voltage density
Chromaticity Luminance efficiency quantum (V) (mA/cm.sup.2) x y
(cd/m.sup.2) (cd/A) efficiency(%) Light-emitting 2.7 1.5 0.56 0.44
1097 71 27 element 2
[0318] FIG. 23 shows an emission spectrum at a current density of
the light-emitting element 2 of 2.5 mA/cm.sup.2. As shown in FIG.
23, the emission spectrum of the light-emitting element 2 has a
peak at 584 nm.
[0319] As shown in Table 4, the CIE chromaticity coordinates of the
light-emitting element 2 at a luminance of 1097 cd/m.sup.2 were (x,
y)=(0.56, 0.44). The results show that light emanating from the
dopant was obtained.
[0320] As described above, the light-emitting element 2 which
includes the light-emitting layer containing the phosphorescent
compound and PCBAPDBq which is the organic compound of one
embodiment of the present invention can efficiently emit light in
the orange wavelength range. It can be said that PCBAPDBq is
suitable as an assist material for a material emitting light with a
wavelength equal to or longer than that of orange light.
[0321] Next, a reliability test was performed on the light-emitting
element 2. FIG. 24 shows results of the reliability test.
[0322] In the reliability test, the light-emitting element 2 was
driven under the conditions where the initial luminance was 5000
cd/m.sup.2 and the current density was constant. The results are
shown in FIG. 24. The horizontal axis represents driving time (h)
of the element and the vertical axis represents normalized
luminance (%) on the assumption that the initial luminance is 100%.
According to FIG. 24, it takes about 1000 hours for the normalized
luminance of the light-emitting element 2 to decline to 72%.
[0323] FIG. 24 shows that the light-emitting element 2 has a long
lifetime.
[0324] The above results show that the light-emitting element 2
which includes the light-emitting layer containing the
phosphorescent compound and PCBAPDBq which is the organic compound
of one embodiment of the present invention has high efficiency, low
driving voltage, low power consumption, and a long lifetime.
Example 5
[0325] In this example, a light-emitting element 3 which includes a
light-emitting layer containing a phosphorescent compound and the
organic compound which is described in Embodiment 1 and Example 1
and is represented by Structural Formula (100) and a comparative
light-emitting element 4 for comparison were evaluated. Chemical
formulae of materials used in this example are shown below.
##STR00064## ##STR00065##
[0326] First, the light-emitting element 3 is described with
reference to FIG. 12. A manufacturing method of the light-emitting
element 3 of this example is described below.
(Light-Emitting Element 3)
[0327] First, an indium oxide-tin oxide compound containing silicon
or silicon oxide (ITSO) was deposited by a sputtering method over a
substrate 1100, whereby a first electrode 1101 was formed. Note
that the composition ratio of In.sub.2O.sub.3 to SnO.sub.2 and
SiO.sub.2 in the target used was 85:10:5 [wt %]. The thickness of
the first electrode 1101 was 110 nm and the electrode area was 2
mm.times.2 mm. Here, the first electrode 1101 functions as an anode
of the light-emitting element.
[0328] Next, as pretreatment for forming the light-emitting element
over the substrate 1100, the surface of the substrate was washed,
baked at 200.degree. C. for one hour, and subjected to UV ozone
treatment for 370 seconds.
[0329] After that, the substrate 1100 was transferred into a vacuum
evaporation apparatus where the pressure had been reduced to
approximately 10.sup.-4 Pa, and subjected to vacuum baking at
170.degree. C. for 30 minutes in a heating chamber of the vacuum
evaporation apparatus. Then, the substrate 1100 was cooled down for
about 30 minutes.
[0330] Next, the substrate 1100 provided with the first electrode
1101 was fixed to a substrate holder in the vacuum evaporation
apparatus so that a surface on which the first electrode 1101 was
provided faced downward. The pressure in the vacuum evaporation
apparatus was reduced to about 10.sup.-4 Pa. Then, DBT3P-II
(abbreviation) and molybdenum oxide were co-evaporated to form a
hole-injection layer 1111 on the first electrode 1101. The
thickness of the hole-injection layer 1111 was set to 30 nm, and
the weight ratio of DBT3P-II to molybdenum oxide was adjusted to
4:2 (=DBT3P-II: molybdenum oxide).
[0331] Next, on the hole-injection layer 1111, a film of BPAFLP
(abbreviation) was formed to a thickness of 20 nm, whereby a
hole-transport layer 1112 was formed.
[0332] Next, PCBAPDBq (abbreviation) synthesized in Example 1 and
(dipivaloylmethanato)bis(2,3,5-triphenylpyrazinato)iridium(III)
(abbreviation: Ir(tppr).sub.2(dpm)) were co-evaporated, whereby a
light-emitting layer 1113 was formed on the hole-transport layer
1112. Here, the weight ratio of PCBAPDBq to Ir(tppr).sub.2(dpm) was
adjusted to 1:0.05 (=PCBAPDBq: Ir(tppr).sub.2(dpm)). The thickness
of the light-emitting layer 1113 was set to 40 nm.
[0333] Note that Ir(tppr).sub.2(dpm) is a phosphorescent compound
and a guest material (dopant) in the light-emitting layer 1113.
Further, PCBAPDBq is a host material in the light-emitting layer
1113.
[0334] Next, a film of PCBAPDBq was formed to a thickness of 20 nm
on the light-emitting layer 1113, whereby a first
electron-transport layer 1114a was formed.
[0335] Next, a film of BPhen (abbreviation) was formed to a
thickness of 20 nm on the first electron-transport layer 1114a,
whereby a second electron-transport layer 1114b was formed.
[0336] Next, a film of lithium fluoride (LiF) was formed to a
thickness of 1 nm on the second electron-transport layer 1114b by
evaporation, whereby an electron-injection layer 1115 was
formed.
[0337] Lastly, a film of aluminum was formed to a thickness of 200
nm by evaporation, whereby a second electrode 1103 functioning as a
cathode was formed. Thus, the light-emitting element 3 of this
example was manufactured.
(Comparative Light-Emitting Element 4)
[0338] As for the comparative light-emitting element 4,
4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBA1BP) and Ir(tppr).sub.2(dpm) (abbreviation) were
co-evaporated, whereby a light-emitting layer 1113 was formed on
the hole-transport layer 1112. Here, the weight ratio of PCBA1BP to
Ir(tppr).sub.2(dpm) was adjusted to 1:0.05 (.dbd.PCBA1BP:
Ir(tppr).sub.2(dpm)). The thickness of the light-emitting layer
1113 was set to 40 nm.
[0339] Note that the comparative light-emitting element 4 was
manufactured in a manner similar to that of the light-emitting
element 3 other than the light-emitting layer 1113.
[0340] Table 5 shows element structures of the light-emitting
element 3 and the comparative light-emitting element 4 which were
obtained as described above.
TABLE-US-00005 TABLE 5 Hole- Hole- Electron- First injection
transport First electron- Second electron- injection Second
electrode layer layer Light-emitting layer transport layer
transport layer layer electrode Light- ITSO DBT3P- BPAFLP
PCBAPDBq:Ir(tppr).sub.2(dpm) PCBAPDBq BPhen LiF Al emitting 110 nm
II:MoOx 20 nm (=1:0.05) 20 nm 20 nm 1 nm 200 nm element 3 (=4:2) 40
nm 30 nm Comparative ITSO DBT3P- BPAFLP PCBA1BP:Ir(tppr).sub.2(dpm)
PCBAPDBq BPhen LiF Al light- 110 nm II:MoOx 20 nm (=1:0.05) 20 nm
20 nm 1 nm 200 nm emitting (=4:2) 40 nm element 4 30 nm
[0341] Next, in a glove box containing a nitrogen atmosphere, the
light-emitting element 3 and the comparative light-emitting element
4 were sealed so as not to be exposed to the air (a sealant was
applied onto an outer edge of the element and heat treatment was
performed at 80.degree. C. for one hour at the time of sealing).
After that, operation characteristics of the light-emitting element
3 and the comparative light-emitting element 4 were measured. Note
that the measurement was carried out at room temperature (in an
atmosphere kept at 25.degree. C.).
[0342] FIG. 30 shows luminance versus current density
characteristics of the light-emitting element 3 and the comparative
light-emitting element 4. In FIG. 30, the horizontal axis
represents current density (mA/cm.sup.2) and the vertical axis
represents luminance (cd/m.sup.2). FIG. 31 shows luminance versus
voltage characteristics of the light-emitting element 3 and the
comparative light-emitting element 4. In FIG. 31, the horizontal
axis represents voltage (V) and the vertical axis represents
luminance (cd/m.sup.2). FIG. 32 shows current efficiency versus
luminance characteristics of the light-emitting element 3 and the
comparative light-emitting element 4. In FIG. 32, the horizontal
axis represents luminance (cd/m.sup.2) and the vertical axis
represents current efficiency (cd/A). FIG. 33 shows current versus
voltage characteristics of the light-emitting element 3 and the
comparative light-emitting element 4. In FIG. 33, the horizontal
axis represents voltage (V) and the vertical axis represents
current (mA).
[0343] FIG. 30 and FIG. 32 show that the light-emitting element 3
has higher efficiency than the comparative light-emitting element
4. FIG. 30, FIG. 31, and FIG. 33 show that the light-emitting
element 3 has lower driving voltage and lower power consumption
than the comparative light-emitting element 4.
[0344] Table 6 shows the voltage (V), current density
(mA/cm.sup.2), CIE chromaticity coordinates (x, y), luminance
(cd/m.sup.2), current efficiency (cd/A), and external quantum
efficiency (%) of the light-emitting element 3 and the comparative
light-emitting element 4 at a luminance of around 1000
cd/m.sup.2.
TABLE-US-00006 TABLE 6 Current Current External Voltage density
Chromaticity Luminance efficiency quantum (V) (mA/cm.sup.2) x y
(cd/m.sup.2) (cd/A) efficiency(%) Light-emitting 2.9 4.6 0.65 0.34
1152 25 21 element 3 Comparative 3.9 8.8 0.44 0.46 1029 12 5
light-emitting element 4
[0345] As shown in Table 6, the CIE chromaticity coordinates of the
light-emitting element 3 at a luminance of 1152 cd/m.sup.2 were (x,
y)=(0.65, 0.34). Further, the CIE chromaticity coordinates of the
comparative light-emitting element 4 at a luminance of 1029
cd/m.sup.2 were (x, y)=(0.44, 0.46). The results show that light
emanating from the dopant was obtained from the light-emitting
element 3 of one embodiment of the present invention.
[0346] As described above, the light-emitting element 3 which
includes the light-emitting layer containing the phosphorescent
compound and PCBAPDBq which is the organic compound of one
embodiment of the present invention can efficiently emit light in
the red wavelength range. It can be said that PCBAPDBq is suitable
as a host material for a material emitting light with a wavelength
equal to or longer than that of orange light.
[0347] Next, reliability tests were performed on the light-emitting
element 3 and the comparative light-emitting element 4. FIG. 34
shows results of the reliability test.
[0348] In the reliability test, the light-emitting element 3 and
the light-emitting element 4 were driven under the conditions where
the initial luminance was 5000 cd/m.sup.2 and the current density
was constant. The results are shown in FIG. 34. The horizontal axis
represents driving time (h) of the element and the vertical axis
represents normalized luminance (%) on the assumption that the
initial luminance is 100%. According to FIG. 34, it takes about 97
hours for the normalized luminance of the light-emitting element 3
to decline to 80%. Further, it takes about 95 hours for the
normalized luminance of the comparative light-emitting element 4 to
decline to 50%.
[0349] FIG. 34 shows that the light-emitting element 3 has a longer
lifetime than the comparative light-emitting element 4.
[0350] The above results show that the light-emitting element 3
which includes the light-emitting layer containing the
phosphorescent compound and PCBAPDBq which is the organic compound
of one embodiment of the present invention has high efficiency, low
driving voltage, low power consumption, and a long lifetime.
Example 6
[0351] In this example, a light-emitting element 5 which includes a
light-emitting layer containing a phosphorescent compound and the
organic compound which is described in Embodiment 1 and Example 1
and is represented by Structural Formula (100) and a comparative
light-emitting element 6 for comparison were evaluated. Chemical
formulae of materials used in this example are shown below.
##STR00066## ##STR00067##
[0352] First, the light-emitting element 5 is described with
reference to FIG. 12. A manufacturing method of the light-emitting
element 5 of this example is described below.
(Light-Emitting Element 5)
[0353] First, an indium oxide-tin oxide compound containing silicon
or silicon oxide (ITSO) was deposited by a sputtering method over a
substrate 1100, whereby a first electrode 1101 was formed. Note
that the composition ratio of In.sub.2O.sub.3 to SnO.sub.2 and
SiO.sub.2 in the target used was 85:10:5 [wt %]. The thickness of
the first electrode 1101 was 110 nm and the electrode area was 2
mm.times.2 mm. Here, the first electrode 1101 functions as an anode
of the light-emitting element.
[0354] Next, as pretreatment for forming the light-emitting element
over the substrate 1100, the surface of the substrate was washed,
baked at 200.degree. C. for one hour, and subjected to UV ozone
treatment for 370 seconds.
[0355] After that, the substrate 1100 was transferred into a vacuum
evaporation apparatus where the pressure had been reduced to
approximately 10.sup.-4 Pa, and subjected to vacuum baking at
170.degree. C. for 30 minutes in a heating chamber of the vacuum
evaporation apparatus. Then, the substrate 1100 was cooled down for
about 30 minutes.
[0356] Next, the substrate 1100 provided with the first electrode
1101 was fixed to a substrate holder in the vacuum evaporation
apparatus so that a surface on which the first electrode 1101 was
provided faced downward. The pressure in the vacuum evaporation
apparatus was reduced to about 10.sup.-4 Pa. Then, DBT3P-II
(abbreviation) and molybdenum oxide were co-evaporated to form a
hole-injection layer 1111 on the first electrode 1101. The
thickness of the hole-injection layer 1111 was set to 40 nm, and
the weight ratio of DBT3P-II to molybdenum oxide was adjusted to
4:2 (=DBT3P-II: molybdenum oxide).
[0357] Next, on the hole-injection layer 1111, a film of BPAFLP
(abbreviation) was formed to a thickness of 20 nm, whereby a
hole-transport layer 1112 was formed.
[0358] Next, 2mDBTPDBq-II (abbreviation), PCBAPDBq (abbreviation)
synthesized in Example 1, and [Ir(dppm).sub.2(acac)] (abbreviation)
were co-evaporated, whereby a light-emitting layer 1113 was formed
on the hole-transport layer 1112. Here, the weight ratio of
2mDBTPDBq-II to PCBAPDBq and [Ir(dppm).sub.2(acac)] was adjusted to
0.8:0.2:0.05 (=2mDBTPDBq-II: PCBAPDBq: [Ir(dppm).sub.2(acac)]). The
thickness of the light-emitting layer 1113 was set to 40 nm.
[0359] Note that [Ir(dppm).sub.2(acac)] is a phosphorescent
compound and a guest material (dopant) in the light-emitting layer
1113. Further, 2mDBTPDBq-II is a host material in the
light-emitting layer 1113. Further, PCBAPDBq is an assist material
in the light-emitting layer 1113.
[0360] Next, a film of 2mDBTPDBq-II was formed to a thickness of 10
nm on the light-emitting layer 1113, whereby a first
electron-transport layer 1114a was formed.
[0361] Next, a film of BPhen (abbreviation) was formed to a
thickness of 20 nm on the first electron-transport layer 1114a,
whereby a second electron-transport layer 1114b was formed.
[0362] Next, a film of lithium fluoride (LiF) was formed to a
thickness of 1 nm on the second electron-transport layer 1114b by
evaporation, whereby an electron-injection layer 1115 was
formed.
[0363] Lastly, a film of aluminum was formed to a thickness of 200
nm by evaporation, whereby a second electrode 1103 functioning as a
cathode was formed. Thus, the light-emitting element 5 of this
example was manufactured.
(Comparative Light-Emitting Element 6)
[0364] As for the comparative light-emitting element 6,
2mDBTPDBq-II,
4-(9-phenyl-9H-carbazol-3-yl)-4'-(3-phenylqunoxalin-2-yl)triphenylamine
(abbreviation: PCBA1PQ), and [Ir(dppm).sub.2(acac)] were
co-evaporated, whereby a light-emitting layer 1113 was formed on
the hole-transport layer 1112. Here, the weight ratio of
2mDBTPDBq-II to PCBA1PQ and [Ir(dppm).sub.2(acac)] was adjusted to
0.8:0.2:0.05 (=2mDBTPDBq-II: PCBA1PQ: [Ir(dppm).sub.2(acac)]). The
thickness of the light-emitting layer 1113 was set to 40 nm.
[0365] Note that the comparative light-emitting element 6 was
manufactured in a manner similar to that of the light-emitting
element 5 other than the light-emitting layer 1113.
[0366] Table 7 shows element structures of the light-emitting
element 5 and the comparative light-emitting element 6 which were
obtained as described above.
TABLE-US-00007 TABLE 7 Hole- Hole- Electron- First injection
transport First electron- Second electron- injection Second
electrode layer layer Light-emitting layer transport layer
transport layer layer electrode Light- ITSO DBT3P- BPAFLP
2mDBTPDBq- 2mDBTPDBq-II BPhen LiF Al emitting 110 nm II:MoOx 20 nm
II:PCBAPDBq:Ir(dppm).sub.2acac 10 nm 20 nm 1 nm 200 nm element 5
(=4:2) (=0.8:0.2:0.05) 40 nm 40 nm Comparative ITSO DBT3P- BPAFLP
2mDBTPDBq- 2mDBTPDBq-II BPhen LiF Al light- 110 nm II:MoOx 20 nm
II:PCBA1PQ:Ir(dppm).sub.2acac 10 nm 20 nm 1 nm 200 nm emitting
(=4:2) (=0.8:0.2:0.05) element 6 30 nm 40 nm
[0367] Next, in a glove box containing a nitrogen atmosphere, the
light-emitting element 5 and the comparative light-emitting element
6 were sealed so as not to be exposed to the air (a sealant was
applied onto an outer edge of the element and heat treatment was
performed at 80.degree. C. for one hour at the time of sealing).
After that, operation characteristics of the light-emitting element
5 and the comparative light-emitting element 6 were measured. Note
that the measurement was carried out at room temperature (in an
atmosphere kept at 25.degree. C.).
[0368] FIG. 35 shows luminance versus current density
characteristics of the light-emitting element 5 and the comparative
light-emitting element 6. In FIG. 35, the horizontal axis
represents current density (mA/cm.sup.2) and the vertical axis
represents luminance (cd/m.sup.2). FIG. 36 shows luminance versus
voltage characteristics of the light-emitting element 5 and the
comparative light-emitting element 6. In FIG. 36, the horizontal
axis represents voltage (V) and the vertical axis represents
luminance (cd/m.sup.2). FIG. 37 shows current efficiency versus
luminance characteristics of the light-emitting element 5 and the
comparative light-emitting element 6. In FIG. 37, the horizontal
axis represents luminance (cd/m.sup.2) and the vertical axis
represents current efficiency (cd/A). FIG. 38 shows current versus
voltage characteristics of the light-emitting element 5 and the
comparative light-emitting element 6. In FIG. 38, the horizontal
axis represents voltage (V) and the vertical axis represents
current (mA).
[0369] FIG. 37 shows that the light-emitting element 5 has higher
current efficiency than the comparative light-emitting element
6.
[0370] Table 8 shows the voltage (V), current density
(mA/cm.sup.2), CIE chromaticity coordinates (x, y), luminance
(cd/m.sup.2), current efficiency (cd/A), and external quantum
efficiency (%) of the light-emitting element 5 and the comparative
light-emitting element 6 at a luminance of around 1000
cd/m.sup.2.
TABLE-US-00008 TABLE 8 Current Current External Voltage density
Chromaticity Luminance efficiency quantum (V) (mA/cm.sup.2) x y
(cd/m.sup.2) (cd/A) efficiency(%) Light-emitting 2.8 1.5 0.56 0.43
1101 72 28 element 5 Comparative 2.8 1.5 0.56 0.43 1008 69 28
light-emitting element 6
[0371] As shown in Table 8, the CIE chromaticity coordinates of the
light-emitting element 5 at a luminance of 1101 cd/m.sup.2 were (x,
y)=(0.56, 0.43). Further, the CIE chromaticity coordinates of the
comparative light-emitting element 6 at a luminance of 1008
cd/m.sup.2 were (x, y)=(0.56, 0.43). The results show that light
emanating from the dopant was obtained.
[0372] As described above, the light-emitting element 5 which
includes the light-emitting layer containing the phosphorescent
compound and PCBAPDBq which is the organic compound of one
embodiment of the present invention can efficiently emit light in
the orange wavelength range. It can be said that PCBAPDBq is
suitable as an assist material for a material emitting light with a
wavelength equal to or longer than that of orange light.
[0373] Next, reliability tests were performed on the light-emitting
element 5 and the comparative light-emitting element 6. FIG. 39
shows results of the reliability test.
[0374] In the reliability test, the light-emitting element 5 and
the comparative light-emitting element 6 were driven under the
conditions where the initial luminance was 5000 cd/m.sup.2 and the
current density was constant. The results are shown in FIG. 39. The
horizontal axis represents driving time (h) of the element and the
vertical axis represents normalized luminance (%) on the assumption
that the initial luminance is 100%. According to FIG. 39, it takes
about 790 hours for the normalized luminance of the light-emitting
element 5 to decline to 73%. Further, it takes about 510 hours for
the normalized luminance of the comparative light-emitting element
6 to decline to 70%.
[0375] FIG. 39 shows that the light-emitting element 5 has a longer
lifetime than the comparative light-emitting element 6.
[0376] The above results show that the light-emitting element 5
which includes the light-emitting layer containing the
phosphorescent compound and PCBAPDBq which is the organic compound
of one embodiment of the present invention has high efficiency and
a long lifetime.
Example 7
[0377] In this example, a light-emitting element 7 which includes a
light-emitting layer containing a phosphorescent compound and the
organic compound which is described in Embodiment 1 and Example 2
and is represented by Structural Formula (201) was evaluated.
Chemical formulae of materials used in this example are shown
below.
##STR00068## ##STR00069##
[0378] The light-emitting element 7 is described with reference to
FIG. 12. A manufacturing method of the light-emitting element 7 of
this example is described below.
(Light-Emitting Element 7)
[0379] First, an indium oxide-tin oxide compound containing silicon
or silicon oxide (ITSO) was deposited by a sputtering method over a
substrate 1100, whereby a first electrode 1101 was formed. Note
that the composition ratio of In.sub.2O.sub.3 to SnO.sub.2 and
SiO.sub.2 in the target used was 85:10:5 [wt %]. The thickness of
the first electrode 1101 was 110 nm and the electrode area was 2
mm.times.2 mm. Here, the first electrode 1101 functions as an anode
of the light-emitting element.
[0380] Next, as pretreatment for forming the light-emitting element
over the substrate 1100, the surface of the substrate was washed,
baked at 200.degree. C. for one hour, and subjected to UV ozone
treatment for 370 seconds.
[0381] After that, the substrate 1100 was transferred into a vacuum
evaporation apparatus where the pressure had been reduced to
approximately 10.sup.-4 Pa, and subjected to vacuum baking at
170.degree. C. for 30 minutes in a heating chamber of the vacuum
evaporation apparatus. Then, the substrate 1100 was cooled down for
about 30 minutes.
[0382] Next, the substrate 1100 provided with the first electrode
1101 was fixed to a substrate holder in the vacuum evaporation
apparatus so that a surface on which the first electrode 1101 was
provided faced downward. The pressure in the vacuum evaporation
apparatus was reduced to about 10.sup.-4 Pa. Then, DBT3P-II
(abbreviation) and molybdenum oxide were co-evaporated to form a
hole-injection layer 1111 on the first electrode 1101. The
thickness of the hole-injection layer 1111 was set to 20 nm, and
the weight ratio of DBT3P-II to molybdenum oxide was adjusted to
4:2 (=DBT3P-II: molybdenum oxide).
[0383] Next, on the hole-injection layer 1111, a film of BPAFLP
(abbreviation) was formed to a thickness of 20 nm, whereby a
hole-transport layer 1112 was formed.
[0384] Next, mYGAPDBq (abbreviation) synthesized in Example 2,
PCBNBB (abbreviation), and Ir(tppr).sub.2(dpm) (abbreviation) were
co-evaporated, whereby a light-emitting layer 1113 was formed on
the hole-transport layer 1112. Here, the weight ratio of mYGAPDBq
to PCBNBB and Ir(tppr).sub.2(dpm) was adjusted to 0.8:0.2:0.05
(=mYGAPDBq: PCBNBB: Ir(tppr).sub.2(dpm)). The thickness of the
light-emitting layer 1113 was set to 30 nm.
[0385] Note that Ir(tppr).sub.2(dpm) is a phosphorescent compound
and a guest material (dopant) in the light-emitting layer 1113.
Further, mYGAPDBq is a host material in the light-emitting layer
1113. Further, PCBNBB is an assist material in the light-emitting
layer 1113.
[0386] Next, on the light-emitting layer 1113, a film of mYGAPDBq
was formed to a thickness of 25 nm, whereby a first
electron-transport layer 1114a was formed.
[0387] Next, a film of BPhen (abbreviation) was formed to a
thickness of 25 nm on the first electron-transport layer 1114a,
whereby a second electron-transport layer 1114b was formed.
[0388] Next, a film of lithium fluoride (LiF) was formed to a
thickness of 1 nm on the second electron-transport layer 1114b by
evaporation, whereby an electron-injection layer 1115 was
formed.
[0389] Lastly, a film of aluminum was formed to a thickness of 200
nm by evaporation, whereby a second electrode 1103 functioning as a
cathode was formed. Thus, the light-emitting element 7 of this
example was manufactured.
[0390] Table 9 shows an element structure of the light-emitting
element 7 obtained as described above.
TABLE-US-00009 TABLE 9 Hole- Hole- Electron- First injection
transport First electron- Second electron- injection Second
electrode layer layer Light-emitting layer transport layer
transport layer layer electrode Light- ITSO DBT3P- BPAFLP
mYGAPDBq:PCBNBB: mYGAPDBq BPhen LiF Al emitting 110 nm II:MoOx 20
nm Ir(tppr).sub.2(dpm) 25 nm 25 nm 1 nm 200 nm element 7 (=4:2)
(=0.8:0.2:0.05) 20 nm 30 nm
[0391] Next, in a glove box containing a nitrogen atmosphere, the
light-emitting element 7 was sealed so as not to be exposed to the
air (a sealant was applied onto an outer edge of the element and
heat treatment was performed at 80.degree. C. for one hour at the
time of sealing). After that, operation characteristics of the
light-emitting element 7 were measured. Note that the measurement
was carried out at room temperature (in an atmosphere kept at
25.degree. C.).
[0392] FIG. 40 shows luminance versus current density
characteristics of the light-emitting element 7. In FIG. 40, the
horizontal axis represents current density (mA/cm.sup.2) and the
vertical axis represents luminance (cd/m.sup.2). FIG. 41 shows
luminance versus voltage characteristics of the light-emitting
element 7. In FIG. 41, the horizontal axis represents voltage (V)
and the vertical axis represents luminance (cd/m.sup.2). FIG. 42
shows current efficiency versus luminance characteristics of the
light-emitting element 7. In FIG. 42, the horizontal axis
represents luminance (cd/m.sup.2) and the vertical axis represents
current efficiency (cd/A). FIG. 43 shows current versus voltage
characteristics of the light-emitting element 7. In FIG. 43, the
horizontal axis represents voltage (V) and the vertical axis
represents current (mA).
[0393] FIG. 40 and FIG. 42 show that the light-emitting element 7
has high efficiency. FIG. 40, FIG. 41, and FIG. 43 show that the
light-emitting element 7 has low driving voltage and low power
consumption.
[0394] Table 10 shows the voltage (V), current density
(mA/cm.sup.2), CIE chromaticity coordinates (x, y), luminance
(cd/m.sup.2), current efficiency (cd/A), and external quantum
efficiency (%) of the light-emitting element 7 at a luminance of
672 cd/m.sup.2.
TABLE-US-00010 TABLE 10 Current Current External Voltage density
Chromaticity Luminance efficiency quantum (V) (mA/cm.sup.2) x y
(cd/m.sup.2) (cd/A) efficiency(%) Light-emitting 2.8 1.9 0.65 0.35
672 36 26 element 7
[0395] As shown in Table 10, the CIE chromaticity coordinates of
the light-emitting element 7 at a luminance of 672 cd/m.sup.2 were
(x, y)=(0.65, 0.35). The results show that light emanating from the
dopant was obtained.
[0396] As described above, the light-emitting element 7 which
includes the light-emitting layer containing the phosphorescent
compound and mYGAPDBq which is the organic compound of one
embodiment of the present invention can efficiently emit light in
the red wavelength range. This shows that mYGAPDBq is suitable as a
host material for a light-emitting material.
[0397] This application is based on Japanese Patent Application
serial no. 2011-258031 filed with Japan Patent Office on Nov. 25,
2011, the entire contents of which are hereby incorporated by
reference.
* * * * *